Add option to set from stdin using railway variables

[jzeuzs]

Resolves #635.

[mikewesthad]

Would be nice to handle the case where users forget to pipe, so we don't hang forever. Maybe something like this:

Suggested change

	if args.set_from_stdin {
	if args.set_from_stdin {
	if std::io::stdin().is_terminal() {
	bail!("--set-from-stdin requires input from stdin (e.g., via pipe or redirect)");
	}

[mikewesthad]

Not super important b/c we trim later, but maybe this for consistency

Suggested change

	args.set.push(line);
	args.set.push(line.trim().to_string());

[mikewesthad]

Couple small things but otherwise LGTM. I think being able to do:

echo "TEST=123" | cargo run variables

W/o the flag is a little cleaner, but I think what you have with an explicit flag is probably best here for backwards compat