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

* allow plugins to print help message for implemented commands
* append core rebar.config options to common 'rebar help' message
Support reading mustache 'lists' from files

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}}
Merge pull request #22 from tuncer/help

Implement 'rebar help CMD1 CMD2' and extend common 'rebar help' msg
Merge pull request #59 from Vagabond/adt-read-lists-from-files

Support reading mustache 'lists' from files