Simplify the customization of popover

Created by: fredgate

This is a suggestion I have from a long time. I thought it was missing in version 3, and it might be well to allow it in version 4. When we want to customize the popover (width, content padding...) we actually do not have other choice that to redefine the full template, which is not very readable, maintainable and evolutive. A better option might be to allow to add a class to the popver, so we could add our custom styles to this class without impacting all the popovers.

Example :

.popover-custom .popover-content {
  padding: 0;
}

$('#foo').popover({
  title: ...,
  content: ...,
  class: 'popover-custom'
});