Rebar has support for optional compilers built-in. These compilers might
not be installed causing xref to return errors for the undefined external
function calls query. The updated filter list will catch this error and
act as a nop if they are installed.

Use correct ERL_LIBS separator on Windows.

bin/<release_name> console -kernel test '[{"a","b"}]' fails, because
when $@ is inserted into CMD [{"a","b"}] is interpreted.
Fix the same issue for 'start' command and quote all arguments.

The custom queries are configured in rebar.config via the tuple
{xref_queries, [{query(), query_result()},...]}.  The implementation
passes the query() string to xref:q and compares the return value with
query_result(). It will result in an error if they do not match.

The following configuration, for example, is the same as running the
xref check undefined_function_calls. It additionally filters
ejabberd_logger:*_msg/4 from the result as these functions are generated
on execution by ejabberd and not available at compile time.

{xref_queries, [{"(XC - UC) || (XU - X - B -
                 (\"ejabberd_logger\":\".*_msg\"/\"4\"))",[]}]}.

This patch also modifies the build process of this package by running a
custom query instead of doing a diff against a static xref_warning file.

This change adds the ability to use alternate urls for downloading
dependencies. To make use of alternate urls run:
rebar get-deps alt_urls=true

If you have dir with many erlang app, then you can simple write
{sub_dirs, ["some_dir/*"]}. Rebar will make operation on subdirs of
dir "some_dir".

Fixes basho/rebar#188

The latest version of erlydtl requires that custom tag modules be in
the path when templates are compiled. Without this change rebar silently
exits when using the custom_tags_modules option.

Automatically include modules with callback attributes in
erl_first_files.

* consolidate options
 * add support for building executables