Add a timestamp message pattern placeholder

http://qt-project.org/doc/qt-5/qtglobal.html#qSetMessagePattern shows that there are several placeholders, but one that would also be very useful is a %timestamp placeholder:

2014-10-31T09:57:21.1234

This would make basic profiling of code using existing qCDebug() calls very convenient. For short-lived programs or profiling, it might even make more sense to just provide a %time placeholder:

09:57:21.1234