Add missing response key migration for `html -> body`.

(This was missed in 5186b2b2a8e80cfc05d70420810f082e35442e0e.)

Add tests to ensure response processing for html applies to both the
new `body` field and the deprecated `html` field.

Progress on #24.
Merge pull request #36 from nicksay/migrate-keys

Add missing response key migration for `html -> body`.
Add experimental "unified extraction" for parsing scripts and styles.

Instead of having two methods for parsing scripts and styles, consolidate
them.  This also unifies the parse result object, which will allow
installation of scripts and styles from any fragment in the future.  For now,
guard the new method behind a `experimental-extract-unified` config.

Also, make the parsing logic much more robust, handling both HTML4 and HTML5
formats, as well as oddities like spaces and single quotes.  Add tests.

Progress on #24.