gitweb: support to globally disable a snapshot format

Allow Gitweb administrators to set a 'disabled' key in the
%known_snapshot_formats hash to disable a specific snapshot format.

All formats are enabled by default to maintain backwards compatibility.

Signed-off-by: Mark Rada <marada@uwaterloo.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>