add Discord_ClearPresence() (#104)

send-presence example updated - start a line with 'c' to toggle
whether presence information is sent or not.

Added ClearPresence to .cs bindings

Added ClearPresence to UE4 blueprint class.

examples/unrealstatus/Plugins/discordrpc/Source/discordrpc/Private/DiscordRpcBlueprint.cpp

@@ -145,3 +145,8 @@ void UDiscordRpc::UpdatePresence()
 
     Discord_UpdatePresence(&rp);
 }
+
+void UDiscordRpc::ClearPresence()
+{
+    Discord_ClearPresence();
+}

examples/unrealstatus/Plugins/discordrpc/Source/discordrpc/Public/DiscordRpcBlueprint.h

@@ -108,6 +108,11 @@ public:
               Category = "Discord")
     void UpdatePresence();
 
+    UFUNCTION(BlueprintCallable,
+              meta = (DisplayName = "Clear presence", Keywords = "Discord rpc"),
+              Category = "Discord")
+    void ClearPresence();
+
     UPROPERTY(BlueprintReadOnly,
               meta = (DisplayName = "Is Discord connected", Keywords = "Discord rpc"),
               Category = "Discord")