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}