Asciinema: [Feature request][v2] Please add a `--overwrite` parameter

I've started to give a workout to v2.

One smallish thing, which still is able to annoy me:

1. I want to record to an existing file. However, I do not want to append to it, but to overwrite it. I get a warning asciinema: /path/file.asc already exists, aborting. (This, of course, is a good thing to have -- we do not want to blindly overwrite files.)

2. That means I have to use another shell command (rm /path/file.asc) in between of test recordings. It would be more convenient if I could just add a --overwrite parameter to do that instead, keeping the existing good default to not overwrite existing files.