Use regexp for case-insensitive literal matches

This commit switches back to using the regexp engine for
case-insensitive literal string matches.

This is slower, but at least case-insensitive matches for string
literals will function now. The code is a tiny bit shorter and simpler
too.

Given the aim of the tool is to be useful for ad-hoc searches,
efficiency isn't the concern but rather just getting the job done with
the minimum of fuss / unexpected behaviour.

file.go

@@ -274,12 +274,6 @@ func findMatches(data []byte) (loc []int) {
 			return loc
 		}
 	}
-	for _, s := range searchBytes {
-		pos := bytes.Index(data, s)
-		if pos != -1 {
-			return []int{pos, pos + len(s)}
-		}
-	}
 	return nil
 }