Add custom messages for matchers

This gives users natural, easier to understand text for expectations.

src/spectator/matchers/equality_matcher.cr

@@ -10,6 +10,14 @@ module Spectator
       def match?(expectation : Expectation)
         expectation.actual == @expected
       end
+
+      def message(expectation : Expectation) : String
+        "Expected #{expectation.actual} to equal #{@expected} (using ==)"
+      end
+
+      def negated_message(expectation : Expectation) : String
+        "Expected #{expectation.actual} to not equal #{@expected} (using ==)"
+      end
     end
   end
 end

src/spectator/matchers/matcher.cr

@@ -7,6 +7,8 @@ module Spectator
       end
 
       abstract def match?(expectation : Expectation)
+      abstract def message(expectation : Expectation) : String
+      abstract def negated_message(expectation : Expectation) : String
     end
   end
 end