Support shrinking with bit.ly and j.mp.

To use bit.ly or j.mp you need to specify the host name in ~/.bti with
shrink_host, your bit.ly login with shrink_bitly_login and your bit.ly key with
shrink_bitly_key.  You can register at:

  http://bit.ly/a/sign_up

If you already have an account, you can find your API key at:

  http://bit.ly/a/your_api_key