[CLI] make output to file default for 'keybase prove' flow for large blocks of text

The CLI prove flow outputs a large chunk of data to STDOUT that you are supposed to copy and paste to the service you're trying to prove. (67 lines for a website, 94 lines for github). Unless you have copy and paste working right in your terminal (and let's hope you're not using screen or something like that) it's awkward to get that output in to a form you can upload to github.

Once you realize this problem and run 'keybase prove --help' you discover the -o flag to send the output to a file.

Suggestion: make output to a file the default for large blocks of text (more than 15 lines). Prompt for a filename, allow - for STDOUT. Modify the flags so this is straightforward.

Start by locating the CLI entry point for `keybase prove` and the existing `--help` and `-o` flag handling. Trace how large proof output is currently sent to STDOUT, then define the filename prompt and `-` override behavior; done means large outputs default to a file while explicit STDOUT remains available.

Written by the indexing model from the issue text.