Implement onlyShrinkWhenNeeded

[dalmaer]

If the "onlyShrinkWhenNeeded" setting is true, only shrink URLs when the length of the tweet is getting long.

The use cases are:

• Some users like to see a full descriptive URL if they can, so want this true
• Some users use the tracking ability of a bit.ly, so want this false

[ewpreston]

completo