Allow you to specify that the input to alert.py is html.

Auditors: chris

Test Plan:
Ran
   echo 'hello @csilvers' | env PYTHONPATH=$HOME/khan/webapp ~/khan/third_party/alertlib/alert.py --hipchat="HipChat Tests" --html

and saw it post my message without alerting me (since it only alerts
in text mode, not html mode).