Fix property merging

Much clearer semantics now. All lists are treated as proplists, meaning
we want to:
1) allow duplicates (providers have to avoid them if they must)
2) preserve order of elements that compare equal (`a == {a, val}`)
   through a stable sort (so if `{a, b}` comes before `a`, we keep
   `{a, b}` first in the list
3) In two lists of attributes requiring a merge, we always give the
   'new' profile a priority to override the default one.

Add test case to exercise the eunit provider using the test profile

treat _checkouts as deps that are always compiled

handle additional errors returned from `getopt:parse/2`

support args on tasks passed to 'as'

Fancy profile merge fun

Add one test case to verify the dev_mode option for a release and
another to verify overriding the dev_mode option in a profile for a
release. Verification of proper dev_mode functioning is done in the
rebar_test_utils:check_results/2 function by checking if all the
directories in the release lib dir are symlinks or not and comparing
that result to the dev_mode expectation passed as input to
the check_results function.

With the new priority order, and knowing Relx processes things in
reverse already (possibly building a dict internally), we should flip
our options around to keep them correct.

Rather than using the stdlib lists:umerge, we expand it to allow fuzzy
matching on tuples vs. vals (`key` vs. `{key,val}`) with an overriden
sort order so that two tuples or values comparing equal get a priority
on the newest profile.

This is a partial fix for #287 -- this current patch should be followed
by a relx update to take options in order (as if they were a proplist)
to complete it.

update hg username and email match in templates

include all pkg deps in list of deps returned from update

flatten command to open_port so <17 Erlangs are happy

handle bad package version with error message

Decapitalize short option for 'version' flag

Dep conflict as error

use PRV_ERROR for formattable errors

Also expose it in rebar_api

The option {deps_error_on_conflict, true} will make it so conflicts in
deps being fetched interrupts the operation rather than just display a
warning.

Defaults to `false'.

The deps are sorted and merged, but the merge function merges lists, not
elements. This yields deps that are duplicated and ran for multiple
times.

We first add proper sorts so the keymerge is guaranteed to be fine, and
then do a dedup run to get rid of duplicates if they happen to be.

Fix #267, refactor as/do/namespace interactions

Breaking up initial call to parse from the ones deep inside the provider
parsing to do smarter namespace detection, added 'as' the ability to
look into these also, and cleaned up the code a whole lot that would
depend on implicit assumptions.

A side-effect is that 'do' is now valid for all namespaces, although it
can be overriden.

halt with 0 on successful run

get rid of compile workers

Store base PLT in ~/.cache/rebar3/

set jobs for all compile jobs

merge rebar config opts for parent like done for profiles

Fix typo in error message

fix debug format to avoid crash with custom template

error before:

===> Uncaught error: {'EXIT',
                             {badarg,
                              [{io,format,
                                [<0.23.0>,
                                 "\e[0;36m===> Skipping template ~p, due to presence of a custom template at ~s~n~n\e[0m",
                                 ["/home/mariano/.config/rebar3/templates/fn_app/fn_app.template"]],
                                []},
                               {rebar_templater,prioritize_templates,2,
                                [{file,"src/rebar_templater.erl"},{line,330}]},
                               {rebar_templater,find_templates,1,
                                [{file,"src/rebar_templater.erl"},{line,274}]},
                               {rebar_templater,new,4,
                                [{file,"src/rebar_templater.erl"},{line,47}]},
                               {rebar_prv_new,do,1,
                                [{file,"src/rebar_prv_new.erl"},{line,44}]},
                               {rebar_core,do,2,
                                [{file,"src/rebar_core.erl"},{line,129}]},
                               {rebar3,main,1,
                                [{file,"src/rebar3.erl"},{line,46}]},
                               {escript,run,2,
                                [{file,"escript.erl"},{line,752}]}]}}

Revert "disable smp in escript"