Tidy up from_fen method
Tidy up from_fen method

Realised I could just use regexes to replace characters. The fen.split("
").first feels a little unclear - maybe this should be named.

I'm sure at some point I'm going to be converting these strings to
arrays, but at the moment there's nothing really driving me towards
that. The only thing I need to bear in mind is I don't write things in
such a way as to make that change as easy as possible.

ohthatjames/chess