Add support for keyword arguments throughout API

[Starbuck5]

I think we should be using keyword arguments on any function with more than two parameters that isn't designed to be a 'call me every frame' type of function (e.g. blitting). Probably we discussed this before - but there, I've said it again now.

Posted by Myremylar in #1898 (comment)

That seems like a good policy to me.

[Starbuck5]

Places I've identified to start off

• font.render
• image.load / loadext ? (only two parameters, but it would be weird to do this for image.save but not image.load?)
• image.save / saveext
• image.to(string|bytes)