Support typing messages using $EDITOR

[riking]

For the keybase encrypt and keybase sign commands, when not run with -m to provide a message inline, should:

• if stdin isatty():
  • if $EDITOR is blank:
  • ask the user to select from a sane selection of command-line editors (nano, vim, emacs) and save that to the config file
  • launch the configured editor to write a message to a file in /tmp, created with 0600 permissions in exclusive mode
  • process that file in /tmp
• else: process standard input