Improve file detection with signature check capabilities

This allows more complex detection upon regex rules for a certain amount of
lines.

runtime/syntax/nginx.yaml

@@ -2,7 +2,7 @@ filetype: nginx
 
 detect: 
     filename: "nginx.*\\.conf$|\\.nginx$"
-    header: "^(server|upstream)[a-z ]*\\{$"
+    signature: "^(server|upstream)[a-z ]*\\{$"
 
 rules:
     - preproc: "\\b(events|server|http|location|upstream)[[:space:]]*\\{"