Add ability to filter events by priority/cleanups

Clean up the handling of priority levels within rules. It used to be a
mix of strings handled in various places. Now, in falco_common.h there's
a consistent type for priority-as-number as well as a list of
priority-as-string values. Priorities are passed around as numbers
instead of strings. It's still permissive about capitalization.

Also add the ability to load rules by severity. New falco
config option "priority=<val>"/-o priority=<val> specifies the minimum
priority level of rules that will be loaded.

Add unit tests for same. The test suppresses INFO notifications for a
rule/trace file combination that would otherwise generate them.

userspace/engine/falco_common.cpp

@@ -21,6 +21,16 @@ along with falco.  If not, see <http://www.gnu.org/licenses/>.
 #include "config_falco_engine.h"
 #include "falco_common.h"
 
+std::vector<std::string> falco_common::priority_names = {
+	"Emergency",
+	"Alert",
+	"Critical",
+	"Error",
+	"Warning",
+	"Notice",
+	"Informational",
+	"Debug"};
+
 falco_common::falco_common()
 {
 	m_ls = lua_open();