feat: introduce puppeteer-firefox (#3628)

This adds a proof-of-concept of `puppeteer-firefox`.
This consists of two parts:
- `//experimental/juggler` - patches to apply to Firefox.
- `//experimental/puppeteer-firefox` - front-end code to
be merged with Puppeteer.

As things become more stable, we'll gradually move it out of
the experimental folder.

.eslintignore

@@ -6,3 +6,4 @@ utils/testrunner/examples/
 node6/*
 node6-test/*
 node6-testrunner/*
+experimental/