bugzilla: set URL field

Add a new option -r which allows users to specify a related bug tracker
system name which is used to obtain the last URL from reported_to
element. The obtained url is used in URL field of an new bugzilla bug.

Closes to #189

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Merge pull request #190 from abrt/gh189

post link to retrace page in bugzilla