Add the ability to save off the CSV to a file.

While we are generating the html file also expose a way for a user to
save off a csv file. This prevents having to run the tool twice.

* Add a new flag called csvpath that takes a file path for the csv dump.
* Use csv.writer to simply print formats.
* Modify the dump_csv and count_severity to take a csv writer.

Test: run with a build.log file, compare before and after html/csv dumps.
Change-Id: I27ed730625a352fdc3fb2928433eb5e8b2ea229c