Add decoding support

[andrew-d]

I don't know if this is a thing that will be super-useful, but it'd be interesting to have support for emoji256 -d to turn a sequence of emoji back into bytes. We'll also need to have some way of handling unknown input characters, possibly configurable (ignore, error, replace with 'REPLACEMENT CHARACTER' (U+FFFD)

[ghost]

I don't know, doesn't seem too important to me.