Support start times.

Hello,

I don't know if you want to incorporate this, but I made some changes to support starting videos from somewhere other than the beginning. It appears to work for what I need but let me know if you want me to do a bit more work before you accept it. I've possibly got some things wrong because I've never written PHP before, and know very little about WordPress and YouTube...

I'm using my changes like this:

[embed]http://www.youtube.com/watch?v=H0KzW76Xnjs#t=5m28s [/embed]

Chris.