CMake: Allow to explicitly enable/disable various plugins

Change-Id: Ib9a84b02ee065d9be1eb96152185c47e2e451b15
Reviewed-on: http://review.couchbase.org/43156
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>