Improve file detection with signature check capabilities

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

runtime/syntax/awk.yaml

@@ -2,7 +2,7 @@ filetype: awk
 
 detect:
     filename: "\\.awk$"
-    header: "^#!.*bin/(env +)?awk( |$)"
+    signature: "^#!.*bin/(env +)?awk( |$)"
 
 rules:
     - preproc: "\\$[A-Za-z0-9_!@#$*?\\-]+"