_search_regex function

And here we are with #847!

This is one of those moments when I'm REALLY glad to have tests.
However, I'll look into making WARNINGs fail tests when appropriate (like, when a field thet should be there can't be found).

So, TODO:

- [x] port those two new IEs
- [x] check WARNINGs
- [ ] consider setting a nicer placeholder in output templates for missing fields
- [x] `_html_search_regex` with `cleanHTML`
- [x] *please report this issue on GitHub* on all `ExtractorError`s
- [x] Exceptions on WARNINGs