feat: add WithImports option

[twpayne]

Fixes #31.

@meblum does this look reasonable? You'll need to set the WithImports(false) option in your code.

[meblum]

Wow, you're quick :) From a usability perspective, instead of WithImports(false), perhaps there should be single option that does the equivalent of WithImports(false) WithHeader("") WithPackageName("") ? I don't see the use case of someone setting WithImports(false) but still want a package declaration.

[meblum]

I mean, for the library, that's not a big deal it can stay the way it is, but for the cli it should probably be combined as a single option.

[twpayne]

Good idea. Something like -types-only as in 36cb37a?

[meblum]

Yes! Thanks so much!!!

[twpayne]

OK, done. Tagged v1.6.0 with these extra options and flags. Thank you very much for your work here!