Add flags to rebar_utils:sh to control output and error handling.
Replace calls to os:cmd with calls to rebar_utils:sh.

This patch remedies an issue where the ebin directory would be
erroneously created as a file by the first "mv" command in
rebar_protobuffs_compile.erl [line 106] if the ebin file did not
exist at the root application level.

In essence, the patch ensures that the ebin directory exists at
the application directory level before any "mv" commands are
executed. The following code was inserted at line 106:

    ok = filelib:ensure_dir(filename:join("ebin","dummy")),

Prevent overlooking a compilation warning by removing the target beam
file after fail_on_warning failure.

abnfc is an ABNF parser generator.
Options are:
	- doc_root (defaults to "src")
	- out_dir (defaults to "src")
	- source_ext (defaults to ".abnf")
	- module_ext (defaults to "")

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}}.

mustache:render("{{banan}}", dict:from_list([{banan, true}])).

** exception error: no function clause matching mustache:escape(true,[])
     in function  erl_eval:do_apply/5
     in call from erl_eval:expr/5
     in call from erl_eval:expr/5
     in call from mustache:render/3

- Check the existence of first_files and fail if they are not present
 - Get first_files lists from local instead of inherited config
   definitions, since they only make sense in the local context