Javascript arguments

Splitjoins javascript function arguments.

Implementation is simply done by extracting and reusing functions already present inside `js.vim`.  It might be even more generalizable by moving these helper functions to the main `sj` helper 'module', but this decision I would leave to you, @AndrewRadev.

Also fixes minor issues with semicolon handling in javascript. Splitjoining functions was producing js that wouldn't make it through a linter like `jshint`.

Note that this currently sits on top of #58 and should not be merged. Depending on the fate of #58 I would rebase or cherry-pick this accordingly before taking further action.

Closes #53