Add formatter for js files

[avegancafe]

It's a little rough using vim-airline in javascript, as many many files are called index.js or index.jsx, which means every open file in the tab bar just says index.js or whatever. This PR creates another tabline formatter so that you can see what the parent folder is called if you're editing one of those files. Based off of this gist: https://gist.github.com/valerybugakov/c85dcc802548f6585db3ef3060b293a7
and then changed the default to be the unique_tail_improved formatter that's already in vim-airline

Side note: I realize this is super specific for javascript, but I think it'd be awesome if we had some sort of standard for different languages! I'm sure there are other languages that have this similar problem, maybe we could a. generalize this method so it works in more than just js, or b. have some sort of standard for creating new formatters for different languages

[chrisbra]

the function name is not correct. Please place the function in a new file tabline/formatters/jsformatter.vim

then I think in the documentation should be mentioned that a new formatter is available. Thanks.

[avegancafe]

D'oh. I forgot to rename the file. I'll add a section about tabline formatting to the readme, good call

[avegancafe]

@chrisbra updated the docs as well 👍 lmk what ya think

[avegancafe]

Bump @chrisbra 😄

[chrisbra]

lookgs good, thanks!

[avegancafe]

Awesome, no problem!