cli: add engine flag

This permits switching between the different regex engine modes that
ripgrep supports. The purpose of this flag is to make it easier to
extend ripgrep with additional regex engines.

Closes #1488, Closes #1502

crates/core/args.rs

@@ -582,7 +582,8 @@ impl ArgMatches {
         } else if self.is_present("auto-hybrid-regex") {
             self.matcher_engine("auto", patterns)
         } else {
-            self.matcher_engine("default", patterns)
+            let engine = self.value_of_lossy("engine").unwrap();
+            self.matcher_engine(&engine, patterns)
         }
     }