Add currency lookup helpers

add ? methods to for simple currency selection:
Openalias.addresses('donate@getmonero.org').find(&:xmr?)

lib/openalias.rb

@@ -3,7 +3,16 @@ require 'ostruct'
 
 class Openalias
   VERSION = "0.1.0"
-  class CryptoAddress < OpenStruct; end
+  class CryptoAddress < OpenStruct;
+
+    def method_missing(method_name, *args, &block)
+      if symbol = method_name[/(.*)\?\z/, 1]
+        self.cryptocurrency.to_s.downcase === symbol.to_s.downcase
+      else
+        super
+      end
+    end
+  end
 
   toplevel_dir = File.dirname(File.dirname(__FILE__))
   Thermite::Fiddle.load_module('init_openalias_rust',