Add --last_week/-W option to my_activity.py
Add --last_week/-W option to my_activity.py

Some weeks, writing snippets on Monday just doesn't happen. And during
those weeks, doing date arithmetic then correctly formatting a U.S.
date is too hard.

R=szager@chromium.org, deymo@chromium.org

Review URL: https://codereview.chromium.org/230823003

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@262723 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools