Option to seed the random instance

[josh-perry]

The instance of Random which is used for all randomness should probably be seedable instead of relying on the default constructor:

https://github.com/josh-perry/Tracery.Net/blob/10843cf958a75da9eb456d5f85d2e3d35138e84d/Tracery.Net/Grammar.cs#L28

This means we'll be able to recreate the same output if given the grammar source and seed again.