Adds delete/backspace key. Resolves #93

docs/keycodes.md

@@ -193,6 +193,7 @@
 |`KC.RESET`             |Put the keyboard into DFU mode for flashing                          |
 |`KC.DEBUG`             |Toggle `debug_enabled`, which enables log spew to serial console     |
 |`KC.GESC`              |Escape when tapped, <code>&#96;</code> when pressed with Shift or GUI|
+|`KC.BKDL`              |Backspace when tapped, Delete when pressed with GUI                  |
 |`KC.LEAD`              |The [Leader key]                                                     |
 |`KC.UC_MODE_NOOP`      |Sets UnicodeMode to NOOP                                             |
 |`KC.UC_MODE_LINUX`     |Sets UnicodeMode to Linux                                            |