Add support for more tags in lang file

[pascalchevrel]

Currently we only support two tags:

active

NOTE:...

We need to support generic tags in addition so as to be able to manage page promos on mozilla.org from lang files ex;

lightbeam

webmaker

We need:

• the parser to extract them
• lang_update to not remove them when ran
• current views to not break

Exposing the tags in a separate or existing view is not part of this issue.