Default command name

Implement @leavengood's method for getting the default name for a command.

Closes #335.
I don't think this should be exposed to the python api. (There are other functions that shouldn't be exposed either, but nevermind them for this PR).

Please add "|RegisterWithDefault" to the regexp string at https://github.com/limetext/lime/blob/5e1017aa02fe464005319062f05da839121265ec/tasks/build/gen_python_api.go#L429 to have it ignored.