Improve readable_alternate_candidates for js files

Map between app/assets/javascripts and spec/javascripts so :AV works
properly with javascript specs.

Adapted from @rupurt's patch on an older version of this plugin -
https://github.com/Casecommons/vim-rails/commit/0f6fbd624c2d8c1310d2a79914ad854d9c747e7b
Merge pull request #290 from DanielleSucher/javascript_readable_alternate_candidates

Improve readable_alternate_candidates for js files