Remove Promise polyfill

[invliD]

Instead of bringing in a Promise polyfill, this library should encourage users to bring their own if they need to support old browsers, but shouldn't make this assumption by itself.

In my case I do want that polyfill, but I'm already bringing my own, so this one would just unnecessarily increase my bundle size.

[lgarron]

The main point of this library is that it Just Works™️ across as many browsers as possible, so I want IE to work out of the box.

However, I'd be open to a PR with suitably designed way to opt out of shipping a separate polyfill fallback (note that the polyfill is not used in modern browsers as of #59, so the main remaining concern should be code bloat), e.g.

• a build that outputs two JS files (in a way such that it's easy to use the correct one), or
• avoids loading the polyfill if the module system can already guarantee one.

Examples of similar projects successfully supporting an optional polyfill like this would be useful to know.

[rcchen]

One example of a project that successfully asks people to use their own polyfill is React. From JavaScript Environment Requirements for React:

React 16 depends on the collection types Map and Set. If you support older browsers and devices which may not yet provide these natively (e.g. IE < 11), consider including a global polyfill in your bundled application, such as core-js or babel-polyfill.

Seems fairly reasonable to have a BYOP (bring your own polyfill) stance if you need to support older browsers or are including your own polyfill already (ie. core-js) and don't need a duplicate.

[crazy4groovy]

Obligatory status icon required:
https://twitter.com/acdlite/status/974390255393505280
http://jsbin.com/nevanoheki/1/edit?html,output