Add option to dump HTML warnings per project.

* Use --byproject flag to dump warnings in HTML file by projects.
* Add external/google and vendor/google projects, vs other
  external/non-google and vendor/non-google projects.
* Refactor code in dumpcategory to dump warnings by projects
  when --byproject is specified.
* Recognize new compiler -Wvarargs warning pattern.
* No change to default HTML output format.

Change-Id: I2406ad392ab18683357ff0ee0cf6c41f87cf399c
Test: run through current build.log files.