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

Simple feature, useful to make link to something without copying it.

Add support for defining template variables of the following form:

{variables, [{appid, "mochiwebapp"},
         {author, "Mochi Media <dev@mochimedia.com>"},
         {year, "2010"},
         {version, "0.1"},
         {port, 8080},
         {dest, "{{appid}}"}]}.

Where dest may be overridden on the commandline but will default to
being the appid. Mochiweb uses this so that we can create new
projects from the template in a configurable directory.

So

$ rebar create template=mochiwebapp dest=foo appid=bar

I thought about special casing dest but figured it might be generally
useful to be able to nest template vars.

However this patch only does one level of resolution. So if

{variables, [{foo, "{{bar}}"},
                {bar, "{{foo}}"}]}.

then bar will end up being the literal string {{bar}} and foo the
literal string {{foo}}.

escript:foldl/3 was undocumented and has been replaced with
better APIs post-R13B04. The new exported funs are officially
documented.

usage: {chmod, Mode, File}
where: Mode is an integer, as specified by the 'mode' field of the file_info record
       (see docs for file:write_file_info/2)

{file,In,Out,true} = render with mustache
{file,In,Out,false} = do not render with mustache (leave as-is)

old-form {file,In,Out} is equivalent to {file,In,Out,true}