keys command should have more output formats

[dmolesUC]

Currently the keys command outputs only either raw keys or quoted Go string literals. The first is problematic for keys with newlines or control characters; the second is Go-specific and somewhat confusing for anything involving a backslash; and also still exposes some sequences that can be problematic for viewing in a terminal, such as ANSI color escapes.

Offering strconv.QuoteToASCII as an option would mitigate some of the problems with the latter case, but probably shouldn't be the only additional option. What else makes sense—URL encoding? MIME quoted-printable? Base64?