formatter: include 'cc' in RSS <channel>'s <link>

* Adds collection link to channel if the current collection is defined.
  (addresses #2013)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
search: user-preffered output format

* Introduces a new parameter `of` for personal
  default output format.  (closes #1587)

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>