Validate rule outputs when loading rules.

Validate rule outputs when loading rules by attempting to create a
formatter based on the rule's output field. If there's an error, it will
propagate up through load_rules and cause falco to exit rather than
discover the problem only when trying to format the event and the rule's
output field.

This required moving formats.{cpp,h} into the falco engine directory
from the falco general directory. Note that these functions are loaded
twice in the two lua states used by falco (engine and outputs).

There's also a couple of minor cleanups:

 - falco_formats had a private instance variable that was unused, remove
   it.
 - rename the package for the falco_formats functions to formats instead
   of falco so it's more standalone.
 - don't throw a c++ exception in falco_formats::formatter. Instead
   generate a lua error, which is handled more cleanly.
 - free_formatter doesn't return any values, so set the return value of
   the function to 0.

userspace/engine/formats.h

@@ -0,0 +1,46 @@
+/*
+Copyright (C) 2016 Draios inc.
+
+This file is part of falco.
+
+falco is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License version 2 as
+published by the Free Software Foundation.
+
+falco is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with falco.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#pragma once
+
+#include "sinsp.h"
+
+extern "C" {
+#include "lua.h"
+#include "lualib.h"
+#include "lauxlib.h"
+}
+
+class sinsp_evt_formatter;
+
+class falco_formats
+{
+ public:
+	static void init(sinsp* inspector, lua_State *ls, bool json_output);
+
+	// formatter = falco.formatter(format_string)
+	static int formatter(lua_State *ls);
+
+	// falco.free_formatter(formatter)
+	static int free_formatter(lua_State *ls);
+
+	// formatted_string = falco.format_event(evt, formatter)
+	static int format_event(lua_State *ls);
+
+	static sinsp* s_inspector;
+};