TextMate Tutorial: How to add a Strikethrough keybind to your Markdown bundle
1) Click the + sign on the bottom left and click New Command.
2) Paste this into the editbox and make sure you name your command "Strikethrough".
For the input field, select WORD in the drop down.
For the output field, select "insert as snippet".
As for the keybind, you can totally map it to whatever you're comfortable with but I chose Command-D as it is the same thing in Microsoft Word.