Removed all the options, refactored the code, and fixed a bug:

One of the reasons I wrote this extension was that I found it hard to understand what other password/passphrase extensions were doing. So I decided to make this extension as simple as possible by removing all the options and refactoring the code to make it easier to understand. Hopefully this makes it easy for anyone to verify that it works correctly.

I also fixed a bug where a space was added to the beginning and end of the passphrase.

Added an options page to:
- select the number of words in the passphrase,
- turn on automatic copying of the passphrase to the clipboard.