Support wildcards in lib dirs

Wildcards are supported in both lib_dirs in config files
as well via the -l command line option.
Merge pull request #94 from josevalim/jv-expand-lib-dirs

Support wildcards in lib dirs
Add option to skip release discovery

`rlx_prv_discover` provider recursively scans all directories in
`lib_dirs` list looking for `*.app` and `*.rel` files.

This commit adds an option `disable_rel_discovery` to skip searching for
`*.rel` files when we know there aren't any. This speeds up release
generation with a lot of dependencies on the `libs_dir` list
significantly and reduces total release generation time in half.