Options rules

So far we had a variable to determine the options to be used for `:make`:

    let g:erlang_make_options = '--outdir . --xref'

This PR introduces a variable that is a list of rules so that different options can be used for different directories:

    let g:erlang_make_options_rules =
      \ [{'path_re': 'project_a', 'options': '--outdir . --xref'},
      \  {'path_re': 'project_b', 'options': '--outdir ../ebin'}]

Notes:

- If there is no match, `g:erlang_make_options` is used, so this change is 100% backward compatible.
- Flymake has the same feature: `g:erlang_flymake_options` and `g:erlang_flymake_options_rules`.