Adding Files::register_for_mime_type() to associate a file analyzer

with a MIME type.

Whenever that MIME is detected, Bro will now automatically activate
the analyzer. The interface mimics how well-known ports are defined
for protocol analyzers.

This isn't actually used by any existing file analyzer (because we
don't have any yet that target a specific file format), but there's a
test making sure it works.

src/Func.h

@@ -100,7 +100,7 @@ public:
 protected:
 	BroFunc() : Func(BRO_FUNC)	{}
 	Stmt* AddInits(Stmt* body, id_list* inits);
-	Val* HandlePluginResult(Val* plugin_result, val_list* args);
+	Val* HandlePluginResult(Val* plugin_result, val_list* args) const;
 
 	DECLARE_SERIAL(BroFunc);