Add utility class which adds ellipsis if text overflows

Created by: 0xadri

As explained in the below StackOverflow question, one might often want to add an ellipsis to some text if it overflows.

http://stackoverflow.com/questions/802175/truncating-long-strings-with-css-feasible-yet/22811590#22811590

I created a pull request for this feature https://github.com/twbs/bootstrap/pull/19313

It's my first time contributing to Bootstrap so forgive me if I'm doing anything incorrectly.