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.
`entropyToMnemonic` only takes a single argument, passing `this.wordlist` isn't necessary.