Initial SW stub addition.

Pulling in this for review and discussion.

This is the initial output of a wpt testing program which does the following.

Given a spec shortname it:

1. fetches the spec,
2. splits it up according to section boundaries,
3. generates a stub entry for each section that appears to be testable (heuristics are very basic atm),
4. identifies webIDL blocks,
5. generates idlharness tests for each of those (evens comes up with credible interfaces for missing dependencies using [webidl-deps](https://github.com/tobie/webidl-deps)].

Next steps which are near ready, but some are slightly invasive so wanted to discuss before:

1. prompt mode which simplifies choosing which section is testable and which isn't,
2. auto-open and tag issues for each section (great for distributing work),
3. handles spec changes by opening up issues for each section that has been modified since a test was checked-in.