BIP39: allow for custom wordlists

This pull request removes the concept of "people adding a PR for custom word lists" to this repository, and instead allows for users to provide their own word lists.

This is acceptable since BIP39 is word list agnostic, and as such so is the implementation.
In this assumption, I have also removed all the fussy dependencies that were being used to support this paradigm.

I think the module is better for it.  Thoughts @weilu?