Improve readable_alternate_candidates for js files

Maps 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

(I emailed him to see if he wanted to submit it upstream himself or if I should just do it, and his response was: "Go ahead and submit it yourself. The most important thing is that you fix the issue!" :heart:)