Support reading mustache 'lists' from files

Implement 'rebar help CMD1 CMD2' and extend common 'rebar help' msg

This commit add support for reading mustache 'lists' from files, so you
can use the list section functionality when templating things.

An example of the list syntax is as follows:

{package_commands, {list, [[{name, "riak"}], [{name, "riak-admin"}], [{name, "search-cmd"}]]}}.

Then you can, for each of the list elements, render some text:

{{#package_commands}}
chmod +x bin/{{name}}
{{/package_commands}}

* allow plugins to print help message for implemented commands
* append core rebar.config options to common 'rebar help' message

Fix basho/rebar/issues/286

Support environment vars with unicode characters

Add missing variables

- add overlay_vars=, target_dir=, and tests= with short description

bash-completion: add missing options and variables

- add missing short and long options
- add tests=, target_dir=, overlay_vars=

zsh-completion: add 'dump_spec' back again

bash-completion: add dump_spec=1

rebar.config.sample: fix escript_emu_args sample

Makefile: fix 'clean' and add 'distclean'

Change zsh completion script

bash-completion: add generate-appups

* fix 'clean' to remove .eunit (not .test)
* add 'distclean' to also remove deps/

Update THANKS: use my real name

Fixes

Add missing clean/2

The results returned by os:getenv() may contain unicode characters.
That said, we need to explicitly allow unicode when splitting the
environment information, otherwise badarg will be raised causing all
rebar commands to fail until the environment variable is removed.

app_bin option was used by the install command which has been
removed in 51ed787.

- Add 'escriptize' command
- Remove variable 'dump_spec'

Rename rebar_core functions for improved readability.
Remove redundant comments and blank lines.

Fix rebar.config.sample