Feature: Support default sets

[j6s]

It would be neat if polyfll-injector supported default sets: https://polyfill.io/v2/docs/features/#default-sets

This way one could define polyfills: [ 'default' ] and have a good starting point for further polyfilling.

[j6s]

Similarly it would be nice if we could define groups of polyfills. For example: I am currently using the following polyfills:

• Symbol
• Array.prototype.@@iterator
• NodeList.prototype.@@iterator

In 99.9% of the cases either none of them are loaded or all of them. Allowing them to be grouped would reduce the number of output files.

[SebastianS90]

Moved the second idea to #24