Allow cli/ tool to read data from stdin

[Warbo]

The executable in cli/ is a nice little conversion tool: much lighter than depending on Kubo, and supports more bases than other CLIs I've found. I'm not sure if anybody's actually using it for anything, but I'd like to; however, it does a couple of things which make it unsuitable for scripting. These commits scratch that itch, so I thought I'd upstream them. If it's not appropriate then feel free to ignore me :-)

• Taking data from arguments, but not stdin. This is especially problematic for encode, since binary data (which may contain NUL bytes, etc.) doesn't work well in argument lists; it forces shell scripts to use variables, which can't store it correctly; etc. In contrast, stdio is binary-safe, and requires no variables.
• Outputting more than just the raw data, e.g. including a Result: prefix. This makes it less useful for programmatic use.

With these changes, the multibase executable feels more like a member of coreutils (e.g. base64), and is much easier to plug into other tools.

PS: I'm not a Rust programmer, so might have made some mistakes. It would also be nice to stream the stdio conversion, rather than reading it all into memory; but this is a less invasive change, and fine for my current needs, at least.

[vmx]

Thanks a lot for the PR, that's indeed a nice addition. Sorry for the long delay for the review. The change looks good as it is.

You mentioned that you're new to Rust. Hence I thought I leave a suggestion as I find code reviews a good way to learn from others. It's a matter of style and not about correctness. So feel free to ignore them.

[vmx]

I personally use if let when there's no else (e.g. for early returns). When there's an else, I usually use match.

Suggested change

	let input_bytes = if let Some(s) = input {
	s.into_bytes()
	} else {
	let mut buf = Vec::new();
	io::stdin().read_to_end(&mut buf)?;
	buf
	};
	let input_bytes = match input {
	Some(s) => s.into_bytes(),
	None => {
	let mut buf = Vec::new();
	io::stdin().read_to_end(&mut buf)?;
	buf
	}
	};

It's deeper nested, but I find it easier to read/reason about.

[vmx]

Same reasons as for the comment above:

Suggested change

	let input_str = if let Some(s) = input {
	s
	} else {
	let mut buf = String::new();
	io::stdin().read_to_string(&mut buf)?;
	buf
	};
	let input_str = match input {
	Some(s) => s,
	None => {
	let mut buf = String::new();
	io::stdin().read_to_string(&mut buf)?;
	buf
	}
	};