Add syntax highlighting to README

[ian-h-chamberlain]

This makes the shell commands slightly more readable on Github and the Python snippets much more readable. The only thing I'm not positive about is splitting the Python code from --explain from its shell invocation – it might not be completely clear that the output is from the command, maybe I could add a line in between like Results in: or something?

[hauntsaninja]

Thanks for this and thanks for checking out pyp!

I think it should be okay from context. I reworded things slightly which might help make it a little clearer.