Support --output-format-heading for the headings
('Issues:', etc), and a simple --markdown flag that sets
item and heading formats to something markdown friendly.

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

Review URL: https://codereview.chromium.org/116873002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@243204 0039d316-1c4b-4281-b951-d872f2087c98