Native support for `import`

[judofyr]

Right now we precompile JavaScript to Babel in order to support ES6 import. I'd like to:

• Support import natively in mrmanager
• Don't precompile JS to Babel automatically
• (Possibly) add an option for compiling certain files with Babel

This ties in nicely with the idea of having less magic and being more explicit about what's happening.

Possible issues: Right now .js files can use React. If we change the behaviour now we will break running of old casts.