Adapt Stumpwm's defcommand to Mahogany

[sdilts]

Now that we are able to detect and execute keybindings, we need a way to define and execute commands.

We are still missing any machinery to get prompts from users, so we can't implement everything, but it would be nice to get something working.

Here are the requirements for right now:

• We need to be able to lookup the command via a string or its symbol name.
• Whatever function that gets generated by the decommand macro needs to accept the seat and key sequence that was used to trigger the command as keyword arguments. If the user doesn't want to use this info, it should not affect how they write the command and they shouldn't need to specify those arguments when they write the function signature.
• Any commands registered via decommand should have docstrings and other documentation easily available so we can build a help command.
• The implementation shouldn't expose any non-trivial lexical variables as a part of its interface so it's easier to refactor without breaking a public API.

We don't need all of this at once, but these requirements should guide the initial implementation.