modify `ct` provider to copy selected directories and compile them

alongside their source (ie, if `some_tests` is a directory that
contains test suites beams resulting from compiling them will be
placed in `some_tests` in the appropriate `_build` directory

update the dir used for upgrade deps to the new path under _build

impl shell completion

main module of a plugin just has to export init/1

Support namespace resolution in 'do' provider

cover enhancements

Fixes #306: help text consistency

fix clean all to use proper deps dir

re-export functions only used within rebar_utils that are also needed by...

fix congratulation message in README.md

Fix some dialyzer warnings

generate line by line coverage reports

Polish rough edges of 'new' provider

- Allow 'rebar3 help' to work and have it point to 'rebar3 help
  <template>'
- show a 'template not found' message when a template is indeed not
  found rather than crashing.

to the appropriate `_build` dir

eunit refactor and test suite

matching directory paths

run subsets of test suites

test utils and add better eunit tests

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.