Allow specification of trello card url

This simple PR allows to select a trello card by url during a pull request creation:

``` 
bb gh pr
``` 
Will select first card and will raise exception if more then one in Doing is found

``` 
bb gh pr --card https://trello.com/c/yqHPQhar/1111-my-supercard
``` 
Will select given card from Doing queue and will raise exception if the card is not found in the queue.