Add help option to show valid difficulty levels

[dandrake]

Oh, that's a good idea! Let's make it -d help or similar; special-casing -1 feels just a little clumsy.

Originally posted by @pcantrell in #18 (comment)

[dandrake]

In the above PR I added a bit of help text suggesting that the user can see the range of difficulty types for a given puzzle type by specifying a difficulty of -1, which is invalid for all puzzle types. As suggested, let's not rely on this behavior and include a proper help option or UI.