Adding pushColors to push multiple pixels in one call.

I've been working on faster ways of blitting images onto the screen.  Storing them on an SD card in the correct RGB565 format allows reading bytes from the file and sending them directly to the TFT without needing to do any reformatting.  Doing bulk operations has been key to making that faster.  So far, using a 256 byte buffer (128 pixels), I've sped up drawing a full-screen image from 1 second, to 355ms.
Being able to push multiple pixels/colors at once is the only change needed in the TFT library.