Add fish shell completion script

[playonverbs]

As mentioned by @hedythedev in #2 this adds a completion script for the fish shell. The category file location is read from $HOME/config/, else it defaults to $HOME/stup.

One small issue is the short completion -@, which provides autocompletes for -@n, -@c, -@h and -@v instead.

[ghost]

Oh, still there is something that I forgot: installation instructions — not needed, but would be good.