add missed keys

Hi,

I've added some missed keys that are important in HID scenarios.
Previously I've proposed those changes in deprecated repo of this software and it wasn't applied in current repository as well.
Reference: digistump/DigisparkArduinoIntegration#13

Author: toxydose

digistump-avr/libraries/DigisparkKeyboard/DigiKeyboard.h

@@ -110,7 +110,9 @@ const PROGMEM char usbHidReportDescriptor[USB_CFG_HID_REPORT_DESCRIPTOR_LENGTH]
 #define KEY_0       39
 
 #define KEY_ENTER   40
-
+#define KEY_ESC     41
+#define KEY_BACKSPACE 42
+#define KEY_TAB     43
 #define KEY_SPACE   44
 
 #define KEY_F1      58
@@ -126,6 +128,9 @@ const PROGMEM char usbHidReportDescriptor[USB_CFG_HID_REPORT_DESCRIPTOR_LENGTH]
 #define KEY_F11     68
 #define KEY_F12     69
 
+#define KEY_PRT_SCR 70
+#define KEY_DELETE  76
+
 #define KEY_ARROW_UP        82
 #define KEY_ARROW_DOWN      81
 #define KEY_ARROW_LEFT      80