Add `pyscript serve` subcommand

[mattkram]

Command will run a simple HTTP server in order to host the files dynamically. This is required in order to allow <py-script src="./my_module.py"> to work.

$ pyscript serve my_file.html --show --autoreload

The --autoreload is a nice-to-have, but not necessary right now.

[mattkram]

Also, potentially pyscript serve my_file.py could perform a wrap and then serve in one go.