New customization options for when there are more than max-labels items

[miikka]

I needed to have some extra control over text in the dropdown when there are more than max-labels items, so I made the following changes:

• Make the ellipsis after the items customizable with the attribute more-text, so instead of "foo, bar, ... (5)" you can have "foo, bar (CLICK FOR MORE) (5)"
• Add the attribute show-counter for always hiding the counter ("foo, ... (5)" -> "foo, ...")

I tried to made it so that the defaults are as before. I hope that editing the HTML files was the right way to add the documentation for the new attributes.