Fix interdoc link
Example doc for per-task help
Nuke outdated not-really-doc docs
Fix incorrect examples
DDD for optional flag values, conceptually
Init docstring for Argument
Didn't actually doc how optional flags behave post-parse.
Docstring DD
Skeleton tests
Fix a few basic dumb things
Overhaul new tests to be more comprehensive
WIP
Semi-fix optional flag values
Add explicit set_value API for Argument
Refactor to use new API call yey
Make 1/2 of optional-value tests pass
Fix boog, make other test pass \o/
Merge branch 'master' into per-task-help-53

Conflicts:
	invoke/parser/argument.py
	tests/parser/argument.py
Tiny lang tweak
Refactor optional-value test setup