Allow using solhint as a library and in the browser

[fvictorio]

To test the "library" part: create an npm package, install solhint (you can use yalc for this), and use it like this:

const solhint = require('solhint')

solhint.parseStr('contract Foo {}')

To test the browser part, do the same thing but with a web app. For this to work the webpack configuration must have the node option in true. If you use create-react-app, that is part of the configuration out of the box.

[coveralls]

Coverage remained the same at 96.347% when pulling f204ea4 on solhint-as-library into 88dca20 on 3.0.