Add feature: custom truncation token string

Hi, I added a custom truncation token feature.

You can apply custom truncation token in the following situation:

>NSAttributedString is pretty rad. When it was ported into iOS 4 from Mac OS, iPhone developers everywhere rejoiced. Unfortunately, as of iOS 4 none of the standard controls in UIKit support it. Bummer.
>TTTAttributedLabel was created to be a drop-in replacement for UILabel, that provided a simple API to styling text with NSAttributedString while remaining performant. As a bonus, it also supports link embedding...[More](https://github.com/mattt/TTTAttributedLabel)

Here, the `...More` may be the custom truncation token string