Add passwordstrength pattern

Add new pattern that uses zxcvbn[1] to display a 4-section password strength meter below any input box, obviously a password input box would be a good choice :)

zxcvbn itself isn't included, as it's pretty massive and would undoubtedly ruffle feathers. Instead it is loaded when the pattern first gets used, by default using cdnjs. The plan is that plone.login will include it's own copy instead, so Plone sites don't end up having external dependencies but mockup doesn't get huge.

[1] https://github.com/dropbox/zxcvbn
To properly integrate it, we would also need to add coffeescript compilation to mockup's compilation chain. We'd then add 700k to the mockup JS, which wouldn't make me the most loved Plonista of the month. So it has to be external.

I'm not an AMD expert (and retrofitting zxcvbn isn't a good place to learn) but seemed the only other option was to insert it via. a URL like the above. Given the disconnect between mockup and plone.app.widgets, I don't think there's a sensible Plone URL to hard code whilst still in mockup-land.

Note I don't intend for this URL to actually get used in anger, plone.login should override it with it's own copy of zxcvbn.js