support detecting locally defined polyfills

[amilajack]

This PR recognizes locally defined polyfills by users and doesn't throw errors in those cases.

This is an error (when targeting IE):

const a = new Set();

This PR allows the above to not be an error:

Declaring polyfills:

class Set {} // A user defined polyfill
const a = new Set();

Importing polyfills:

import Set from 'set-polyfill';
const a = new Set();

This PR does not handle the following cases, which will be addressed in later PR's:

const n = navigator; // navigator is supported
n.serviceWorker // navigator.serviceWorker is supported and should throw

• add open collective info to readme
• allow users to locally define polyfills
• add corresponding test cases

Fixes #192
Closes #195