flag: add overridable --help and --version by default

vlib/flag/testdata/simplest_flag_program.v

@@ -0,0 +1,14 @@
+import os
+import flag
+
+fn main() {
+	mut fp := flag.new_flag_parser(os.args)
+	fp.application('abc')
+	fp.version('0.0.1')
+	fp.skip_executable()
+	rest_of_args := fp.finalize() or {
+		eprintln(err)
+		exit(1)
+	}
+	dump(rest_of_args)
+}