Pull web link support into an addon

[Tyriar]

Fixes #1297

---

The main motivation behind this is to slim down the xterm.js API and move the complexity of customizing the web link handler into an addon. To customize it you can now do this:

term.webLinksInit(
  /* handler */ console.log,
  /* options */ {
    validationCallback: (uri, cb) => { console.log('validate', arguments); cb(true); }
  }
);

A nice benefit of doing this is that it makes both the web links and the linkifier seem much simpler as they're now separates into different files.

[mofux]

LGTM 👍