Better $EDITOR handling

[aleeraser]

The current EDITOR handling is a bit lacking.

howdy/howdy/src/cli/config.py

Lines 14 to 20 in aa75c76

	editor = "/bin/nano"
	# Use the user preferred editor if available
	if "EDITOR" in os.environ:
	editor = os.environ["EDITOR"]
	elif os.path.isfile("/etc/alternatives/editor"):
	editor = "/etc/alternatives/editor"

Improvements:

• check that /bin/nano exists before trying to spawn a subprocess
  • or better, search for nano in PATH instead of using a hardcoded path
• use a universally-included editor like vi as default
  • if the goal was to use a more beginner-friendly editor, then perhaps resort to vim only if nano is not available
• suggest using sudo -E howdy [...] in the documentation, as otherwise the command would run in a non-interactive shell (i.e. without sourcing the initialization files), and no environment variable is carried over (including EDITOR) unless explicitly preserved

[boltgolt]

Sounds good, please open a PR