Improve file detection with signature check capabilities

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

runtime/syntax/sed.yaml

@@ -2,7 +2,7 @@ filetype: sed
 
 detect: 
     filename: "\\.sed$"
-    header: "^#!.*bin/(env +)?sed( |$)"
+    signature: "^#!.*bin/(env +)?sed( |$)"
 
 rules:
     - symbol.operator: "[|^$.*+]"