Add --watch flag to build

[cincodenada]

I had heard of picotool's build functionality and hoped it had this feature. No dice, but I was glad to see it was on the list (#25). So, I went ahead and threw together a basic implementation.

This could be polished further, but I think it's pretty functional already. It adds two flags to the build command. --watch[=dir] will cause picotool to sit and watch the given directory (default current directory) for changes. --watch-glob allows specifying what files to watch - by default, it watches *.lua,*.p8,*.png. An obvious improvement would be to watch the files specified, but this was simpler to implement and does the trick for now.

When any change to a watched file happens, it just runs do_build with whatever other arguments are provided, then goes back to watching.

It's mostly a thin wrapper around watchdog, which I found after a brief search. It met my criteria of cross-platform and seeming mature/supported enough, and using it was actually super nice. I made the include optional - so if it is not there, the rest of picotool will work, but --watch will give a nice message saying to install watchdog to enable it. The stub classes to squelch errors in the code that requires it is my design, I don't know if there's some more standard way of doing that.

I'm certainly open to feedback, on substance or style - for one, I still need to add a couple tests for it. Give it a whirl and let me know what you think. After using it for a bit, it does the job but seems a bit overeager...I'll keep tweaking.