Add some non-subset triggers.

Also add an optional integer argument to the deal_damage trigger,
for restricting the trigger to given amounts of damage dealt at once.
Add a 'kicked' adjective.
Add another handful of triggers.
Add a coin flip trigger.
Add support for parsing trigger descriptors.

Trigger descriptors are an additional condition imposed upon a trigger
event to further limit when it occurs, such as "during your turn" or
"while it is exiled". These bind less tightly than the OR in a pair of
triggers.

- Fix parse_triggers to parse the "triggers" rule to correctly test both
  the top-level OR and the trigger descriptors.
- Fix tests to match the triggers change.
- Add some tests for the trigger descriptors.
Change pl_subset to match more.