Add Mt as abbreviation for Matthew

More a note to self than anything else, although if someone wants to write the patch for me I'll gladly commit it.

We could do with "Mt" as an allowable abbrevation for "Matthew". (We already have Matt and Mat). It simply involves changing Mat?t to Ma?t?t in the code.

I'll come back to this some day and address this, but by putting it here I'm making sure I don't forget, and also giving others the chance to help if they wish.