Add functionality to generate final checksum words for a partial mnemonic?

[gavincarr]

Hi Tyler, thanks for your work on this library!

Would you consider it in-scope to include a function to generate a set of valid final checksum words given a partial (N-1) mnemonic? This would be useful for people who are wanting to generate their own seeds using dice or words-out-of-a-hat etc.

If so, would you accept a PR for this? Perhaps an API like:

func ChecksumWords(partialWords []string) ([]string, error)

?

There are python versions of this floating around e.g. https://github.com/avsync/bip39chk, https://github.com/veebch/Bip39-Dice/, but I'm not aware of a Go version.