Adding a UI option to customize error list displayed when showPopover is true

Hi,

This is my first pull request ever so I'm not sure if I'll following the right procedures, so please excuse me if I'm not.

I've been using your library to visually enhance some password textboxes and it's been extremely useful. Almost every scenario that we had in mind before implementing anything have been covered by your library seamlessly. Thanks a lot for the great work! 

However there was a minor requirement that I didn't manage to satisfy with the library as it was: getting rid of the word "Errors" in the list shown in the popover. That's the reason for this pull request, I'm sending you the small change that I did to add another UI option that allows to insert your own function for the list generation.

I hope you'll consider it useful. I'm not a seasoned JavaScript developer, so if you like the idea but you think there is a better way to implement it, please let me know what changes you think that are necessary before including this in the library.

Thanks again for the library.

Looking forward to hearing from you.

Javier.