Use with chaining instead of alternative constructor to override flag characters

src/options.rs

@@ -16,8 +16,10 @@ impl<ID: 'static> Opts<ID> {
   pub const fn new(options: &'static[Opt<ID>]) -> Self {
     Self { flag_chars: "-", options }
   }
-  // TODO: Replace with a parser options construct
-  pub const fn new_flag(flag_chars: &'static str, options: &'static[Opt<ID>]) -> Self {
-    Self { flag_chars, options }
+
+  /// Set the recognised flag/option characters.
+  pub const fn with_flag_chars(mut self, flag_chars: &'static str) -> Self {
+    self.flag_chars = flag_chars;
+    self
   }
 }