Support sixel graphics

[lynlevenick]

While it would move away from being an ASCII renderer, sixel is not entirely unlike a standard for rendering graphics to the terminal.

[sinclairzx81]

Hi, I'd actually really like to support this, however terminals with VT340 emulation support seem to be a bit few and far between (at least for what I've been able to find on Windows), and its probably not supported on most end user terminals. If you can recommend something though, id be happy to take a look, but may have to shelve an implementation for the time being. Sadly had to do the same for ANSI true color for similar reasons.

Of particular note tho, I have been keeping an eye out on microsoft/terminal#448, which may prove of interest should that functionality land.

Thanks for the suggestion :)