Implement Diceware Passphrase Generator

Note: This is a first swing. I do not have the hardware necessary to test it properly at the moment.

Fixes #163 
If, as I commented below, you switch word selection to use this.random(0, this.wordlist.length), you'll also need to update this function to request multiple random bytes. Two bytes (65536 values) initially seems to be sufficient to cover all 7776 possibilities, but I think there wind up being some statistical biases in the rounding. I think for now, grab two bytes and add a TODO to look into fixing biases based on rounding.