Add ability to set the random seed (to make results deterministic)

This commit adds the ability to set the random seed manually when instantiating a Fairy.

This allows you to make the results deterministic; this is useful if you want to get the same names every time you run a Fairy. Say, for example, you have a database of user IDs, but no PII stored with them. You want to create an internal interface that displays metadata about those users, but you don't want to see anything identifying. This allows you to use the user ID as a random seed, thereby *always getting the same fake name for each ID* every time you load the interface.

(Once this is merged, can you tag and release this out? I need to implement this very use case. :))