Allow optional markup of msg2subtitle.
Allow optional markup of msg2subtitle.

This would be nice for embedded datagrepper results so that we can make
links out of people's usernames, update title, badge names, etc..

fedora-infra/fedmsg,chaiku/fedmsg,chaiku/fedmsg,vivekanand1101/fedmsg,mathstuf/fedmsg,fedora-infra/fedmsg,cicku/fedmsg,pombredanne/fedmsg,pombredanne/fedmsg,mathstuf/fedmsg,cicku/fedmsg,mathstuf/fedmsg,vivekanand1101/fedmsg,vivekanand1101/fedmsg,pombredanne/fedmsg,maxamillion/fedmsg,chaiku/fedmsg,fedora-infra/fedmsg,cicku/fedmsg,maxamillion/fedmsg,maxamillion/fedmsg