Allow only alphanum in generated Elm endpoint function names

[rl-king]

Currently generating functions for paths that include - will generate invalid elm function name syntax. This change only allows alpha numeric chars. Which might still be too tolerant, but it filters some of the commonly used symbols in urls.

As I'm writing this I'm wondering if this logic should be part of elm-syntax, any thoughts on that?

[ollef]

It seems like a good idea to handle that here rather than in elm-syntax IMO.

Thanks!