Support kitty and sixel graphics protocol

[vxgmichel]

Mentioned here: #23 (comment)

I haven't bothered with any of the graphics protocols, but if you're going with kitty protocol by the way, I'm feeling like 60fps should be achievable with kitty and sixel covers 33% of terminals, something to consider later. It would also probably benefit from a blitter and get perfect aspect ratio and scale at any height and width.

For the reference, the ucs-detect table

Supporting graphics protocol would be nice, especially since they can be detected:

• blessed.Terminal.does_sixel
• blessed.Terminal.does_kitty_graphics