Make i18n string extraction faster and more flexible

Summary:
Ref T5267. Two general changes:

• Make string extraction use a cache, so that it doesn't take several minutes every time you change something. Minor updates now only take a few seconds (like arc liberate and similar).
• Instead of dumping a sort-of-template file out, write out to a cache (src/.cache/i18n_strings.json). I'm planning to add more steps to read this cache and do interesting things with it (emit translatewiki strings, generate or update standalone translation files, etc).

Test Plan:

• Ran bin/i18n extract.
• Ran it again, saw it go a lot faster.
• Changed stuff, ran it, saw it only look at new stuff.
• Examined caches.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5267

Differential Revision: https://secure.phabricator.com/D16227