Adding support for showSeverityLevel to the HtmlFormatter.

Severity Level is already handled in the TextFormatter. Seems like it was just an oversight in the original code.

showSeverityLevel is false by default and I don't see any code in CodeSearch this will affect. But I need it for a console I'm working on.

R=nicksantos
DELTA=3 (3 added, 0 deleted, 0 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=6085


git-svn-id: http://closure-library.googlecode.com/svn/trunk@2419 0b95b8e8-c90f-11de-9d4f-f947ee5921c8