Allow for function annotations to be used as processing flags.

[amcgregor]

• Basic flag capture. Use of Python 3 function return type annotations will be treated as a comma separated list of flags to add, or, with the presence of an exclamation mark prefix, remove flags from the cinje translation context.

Some ideas for flags:

• Unbuffered. Instead of yielding at breaks and at the end (if the buffer contains anything), with buffer resets and all that nonsense, skip the buffer allocation and local-scope method assignment. Yield at each contiguous block of text, and "yield from" (or fallback equivalent) all nested template use. (Prefix goes from __w(( to yield "".join((.)