watchlist: Add a way to detect a net increase or decrease of a pattern (in a file).

watchlist: Add a way to detect a net increase or decrease of a pattern (in a file).
https://bugs.webkit.org/show_bug.cgi?id=69031

Reviewed by Adam Barth.

* Scripts/webkitpy/common/config/watchlist: Added usage of the new pattern,
a comment to explain something important about the email addresses, and
a comment to let emacs know to treat the file as a python file.
* Scripts/webkitpy/common/watchlist/amountchangedpattern.py: Added.
Detects increases or decreases in a pattern.
* Scripts/webkitpy/common/watchlist/amountchangedpattern_unittest.py: Added.
* Scripts/webkitpy/common/watchlist/watchlist_unittest.py:
Added unit tests for watchlist which has "more" or "less".
* Scripts/webkitpy/common/watchlist/watchlistparser.py:
Added support for "more" or "less".

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96441 268f45cc-cd09-0410-ab3c-d52691b4dbfc