Add monokai theme

[ducaale]

Ticks another item from #4

[blyxxyz]

Almost unreadable in a light terminal, but that's faithful to HTTPie 🙂

Can you add the origin to assets/README.md?

[ducaale]

Just out of curiosity, is the solarized theme usable in a light terminal when using either HTTPie or xh?

[blyxxyz]

It's alright:

But you can't always expect these themes to work with terminal themes they weren't designed for. Unless they just use the basic color codes that are defined by the terminal theme, like the ansi theme does.

HTTPie has a paraiso-dark and a paraiso-light theme, and one is only readable in dark terminals while the other is only readable in light terminals.