Allow for less verbose calling of Generate

[ggutoski]

Currently, the idiomatic way to call bavard.Generate is as follows:

bavard.Generate("pairing/constants.go", src, d,
	bavard.Package("pairing"),
	bavard.Apache2("ConsenSys AG", 2020),
	bavard.GeneratedBy("gurvy/internal/generators"),
)

That's a lot of boilerplate. In particular, the license and GeneratedBy arguments are likely to be identical for every single call.

What's a good way to reduce the boilerplate? Is it acceptable to expose package-level variables for, say, license and "generated by" so that the user can initialize these values once and not have to copy them in every call?