Ask for user input to harden random number generation

When we generate a mnemonic, we should generate random data in the most secure way possible.

Currently [eth-hd-keyring](https://github.com/MetaMask/eth-hd-keyring/blob/master/index.js#L49) uses the `bip39` module's built-in rng, which defaults to browserify's `crypto/getRandomBytes` method, which is the `crypto.getRandomValues` method:
https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues

This seems like one of the best random sources the browser has available, but we should research and verify.

Start by reviewing eth-hd-keyring/index.js around line 49 and the bip39 RNG path, then verify the browser crypto/getRandomValues behavior using the linked MDN documentation. The issue is complete when the security research is documented and a concrete decision is reached about whether and how user input should affect mnemonic randomness.

Written by the indexing model from the issue text.