General purpose flags.

These flags can be used to mark documents as meeting an extensible set
of criteria.
General purpose flags.

These flags can be used to mark documents as
meeting an extensible set of criteria.

Merge branch 'v0.3.0-dev'

Conflicts:
	py/src/streamcorpus/package_globals.py
	py/src/streamcorpus/ttypes.py