Make copy button ignore prompts

[AlbertMitjans]

I would like the copy button to ignore prompts, such as the python prompt (>>>) or the console prompt ($).

Right now these prompts get copied when clicking the copy button.

If this is hard to implement, I would like an option to remove the copy buttons from the code blocks. This way I can use sphinx-copybutton instead.

[kai687]

I thought it wouldn't select console prompts already. I definitely intended it to be this way. So, it's a bug that I'm going to fix.