Allow multiple decorators on the show_urls command

* Allow the user to specify more than one decorator on the show_url command
by passing several "-d" switches on the command line, using the "append" action
of the optparse module instead of the default "store" option.
* Avoid splitting the decorators list on the "table" and "aligned" styles.
Merge pull request #558 from diego-plan9/master

Allow multiple decorators on the show_urls command