Add XBitmap support

Draw XBitMap Files (*.xbm), exported from GIMP,
Usage: 
Export from GIMP to *.xbm, rename *.xbm to *.c and open in editor.
C Array can be directly used with this function
 void Adafruit_GFX::drawXBitmap(int16_t x, int16_t y,
                               const uint8_t *bitmap, int16_t w, int16_t h,
                               uint16_t color)