Bootstrap 4 classes

[YPCrumble]

Bootstrap 4 has new classes for the progress bar background color.

I'd be happy to help with a PR for any of these fixes:

• Updating this repo to support Bootstrap 4 and documenting that Bootstrap 3 users need to add the new Bootstrap 4 classes
• Adding a BOOTSTRAP_VERSION variable that defaults to version 4 but allows the user to set it to version 3 if they're using the old version of bootstrap
• Leaving it the way it is now and just documenting that Bootstrap 4 users need to add the old Bootstrap 3 classes as I've done

Thanks for building this!

[pawamoy]

Hello! Sorry for the delay, I just came back from vacations.

I think the second point is the best. The app would take care of using the right classes depending on a setting variable (BOOTSTRAP_VERSION? or ZXCVBN_PASSWORD_BOOTSTRAP_VERSION?), and the user would just have to update this variable, not the classes themselves.

PR welcome :grin:

[pawamoy]

Will likely not work on this project anymore.