Support for scripting language integrations via plugins. Experimental Lua integration.

- Added program-only plugin support and made UHT script plugin aware
- Added 3 generic script plugins with experimental Lua integration: ScriptGeneratorPlugin (UHT), ScriptPlugin (Engine), ScriptEditorPlugin (Editor-side)
- Lua integration is disabled by default

[CL 2058611 by Robert Manuszewski in Main branch]

Engine/Plugins/Script/ScriptGeneratorPlugin/ScriptGeneratorPlugin.uplugin

@@ -0,0 +1,21 @@
+{
+    "FileVersion" : 3,
+	
+	"FriendlyName" : "Script Generator Plugin",
+	"Version" : 1,
+	"VersionName" : "1.0",
+	"CreatedBy" : "Epic Games, Inc.",
+	"CreatedByURL" : "http://epicgames.com",
+	"EngineVersion" : 1579795,
+	"Description" : "An example of a script generator plugin.  This can be used as a starting point when creating your own plugin.",
+	"Category" : "Examples",
+	
+    "Modules" :
+	[
+		{
+			"Name" : "ScriptGeneratorPlugin",
+			"Type" : "Program",
+			"LoadingPhase" : "PostConfigInit"
+		}
+	]
+}