text options for screen_prompt

`screen_prompt` currently does not allow control of text size, color, etc. (as `screen_text` does). This change addresses that.

There is some repeated code, but I wasn't sure how best to get around that. Let me know what you guys think.
`pos` and `color` were switched. There was also a mismatch in the docs and argument list in both functions, which I have fixed by moving the `units` doc to the end (where the argument is). Not ideal, but done in favor of not breaking old code.