Options rules

[hcs42]

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.