prove-github should write keybase.md to a separate file for gisting

I typed `keybase prove github` without looking at any docs first, and got a wall of text that needs to be pasted as a gist (exactly, as it includes a signature). That's ok, but it would be easier for me if the text got written to a file (perhaps named "keybase.md" as the instructions suggest), from which I could use one of my existing command-line -based tools to gist it, or display it and cut-and-paste if I didn't have one of those.

(maybe there's an option for doing exactly that and I should have checked `--help` before diving in).

Or maybe the tool could display the text, write it to a file, and mention "by the way, this text has been written to ./keybase.md if you'd like to gist it from there" just before asking about "Check github now?".

Also, "Check gist.github.com now?" might be more accurate than just saying github.

Start from the `keybase prove github` command and inspect how its proof text is produced before the “Check github now?” prompt. The change is done when the exact signed text is also written to `keybase.md` and the prompt identifies gist.github.com, while preserving the displayed output.

Written by the indexing model from the issue text.