v3.0.0 - Removing all format_ wrappers, cleanups, and code quality.

[TkTech]

We currently (v1, v2) duplicate all of the babel.dates and babel.numbers format_* methods in Flask-Babel. This is a lot of completely unnecessary duplication, including docstrings and test coverage. We're also often out of date with babel's methods, such as when new kwargs are added. Just like flask-wtf did way back, we should chuck this and become just the glue layer between Flask and Babel. In general, the only change in your code is to add format_decimal(..., locale=get_locale()).

Because this PR reduces our surface area significantly, lets make it a goal to get 100% test coverage.

• Update documentation (Rewrite Documentation #173)
• Remove format helpers (Remove all format_* wrappers #164)
• Remove all old-style formatters in __init__ and test code. (% vs format()). Or bump to py3.6+ and use fstrings.
• Support new-style strings for translation (Add support for new-style Python formatting placeholders #170)
• 100% test coverage, and make it part of the tests.
• 100% flake8, and make it part of the tests.
• Cleanup how we handle translation context. Verify we are thread safe.
• Stop polluting app objects - always use current_app.
• Support multiple apps with different configurations (Incompatible with multiple apps with different translations #107)

The v3.* family aims to be the last breaking release for quite awhile.