Allow arbitrary attributes to be added to commands (#458)

* Allow arbitrary attributes to be added to commands

I still don't approve adding type info back into commands, so
I decided to use this solution for allowing arbitrary attributes to be
added to commands.

Add attributes property to ParameterBuilder

Add Attributes properties to info types

* Why on earth git

* Add using for system so that Attribute can be used

src/Discord.Net.Commands/Builders/ModuleClassBuilder.cs

@@ -122,6 +122,9 @@ namespace Discord.Commands
                     case PreconditionAttribute precondition:
                         builder.AddPrecondition(precondition);
                         break;
+                    default:
+                        builder.AddAttributes(attribute);
+                        break;
                 }
             }
 
@@ -173,6 +176,9 @@ namespace Discord.Commands
                     case PreconditionAttribute precondition:
                         builder.AddPrecondition(precondition);
                         break;
+                    default:
+                        builder.AddAttributes(attribute);
+                        break;
                 }
             }
 
@@ -239,6 +245,9 @@ namespace Discord.Commands
 
                         builder.IsRemainder = true;
                         break;
+                    default:
+                        builder.AddAttributes(attribute);
+                        break;
                 }
             }
 
@@ -289,4 +298,4 @@ namespace Discord.Commands
                    !methodInfo.IsGenericMethod;
         }
     }
-}
+}