`m` with no arguments opens the current directory in TextMate

Copied from https://github.com/mathiasbynens/dotfiles/pull/252

`m` with no arguments opens the current directory in TextMate, otherwise opens the given location. I think the current m alias is inconsistent given that it specifically opens the current directory. But if I want to open just a given file or directory then I have to use `mate mydir`. This function handles both cases and brings consistency with all editor choices.