add `TerminalDisplay` and associated `display` function

[johnnychen94]

#45 allows much clearer image visualization on some terminals(e.g., iTerm2 and mlterm), so with that done, we can start to supporting various image-like objects, e.g., plot, latex, graphics.

To support latex, if I understand it correctly, as long as we provide a TerminalDisplay instance (with displayable(d, MIME("image"png")) == true) to the global Base.Mualtimedia.displays, Latexify.jl would automatically calls it on render. (cc: @korsbo)

https://github.com/korsbo/Latexify.jl/blob/8b39f068874e158b18eac353fff41f9a94c803ef/src/utils.jl#L162-L164

It might be worth mentioning that the pipeline that Latexify.render uses is "luetex --> div2png --> ImageInTerminal", so a significant overhead (0.6s for the minimal example) will be observed even if it's supported.

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I haven't yet tried to figure out what's needed to do to support packages like Plots and Compose, though.