API improvements

[kevva]

Want to make use of Promises and somehow simplify stuff a lot. Also, #31, #37 and #38 needs to be fixed.

[kevva]

@sindresorhus, apart from #37, are there any other things you see can be improved regarding the API?

[sindresorhus]

I wish I didn't have to create this boilerplate file: https://github.com/flowtype/flow-bin/blob/master/lib/install.js

Also wish I could define everything in one file instead of having to have multiple: lib/index.js lib/install.js index.js cli.js. npm exposes through an environment variable when it's run through a postinstall script, so could use that info to have special behavior on install.

Better error handling and messages when something fails.