[TxMt] Help with simple text conversion command

[Michael Reiner]

Can anyone help with a command that would convert the case of selected  
text as well as replaced spaces with underscores?

So, for instance:

This is a Title  ->  this_is_a_title

As a complete beginner in TextMate, I'm not sure how to create a  
command to do this.
Any help would be greatly appreciated.

Best,
Michael R.