Clean up cpp warning patterns

* Remove the useless 'option' key.
  It is only used in some C/C++ warning patterns
  to give a hint of options to turn to -Werror.
  Now the global default is -Werror.
* Factor out common code patterns into high/medium/low functions.

Test: warn.py --url=http://cs/android --separator='?l=' build.log > warnings.html
Test: warn.py --gencsv build.log > warnings.csv
Change-Id: Ibd3f768b1552ada925eb5afb0f01ab674c968a87