gist output to file or pipeable STDOUT

There are numerous command line gist tools. The `keybase prove github` command should be compatible with these tools – the gist content should be spit out to a file or stdout that can be piped to a gist command.

Start at the `keybase prove github` command entry point and inspect how it currently emits gist content. Verify the behavior with file redirection and a pipe to another command; it is done when the gist content can be written to a file or emitted cleanly on STDOUT.

Written by the indexing model from the issue text.