add format param to search api

docs/dev/search_api.rst

@@ -31,3 +31,5 @@ Parameters
 +------------------+----------------------------------------------------------------------------------------------------+--------------------------------------------------+
 | ``time_range``   | Time range of search                                                                               | optional (possible: ``day``, ``month``, ``year``)|
 +------------------+----------------------------------------------------------------------------------------------------+--------------------------------------------------+
+| ``format``       | Output format of results                                                                           | optional (possible: ``json``, ``csv``, ``rss``)  |
++------------------+----------------------------------------------------------------------------------------------------+--------------------------------------------------+