asciicast v2: stored color theme spec

[ku1ik]

We're discussing version 2 of asciicast format in #196, and I think adding optional colors object to the header, containing text foreground/background color and the palette for all 8(16) colors would be a nice addition. It could be saved automatically by asciinema rec and used by web player.

As @mrabault said in asciinema/discussions#8, we can use and update a technique of obtaining the colors, as long as the palette representation in asciicast header stays the same/compatible with the web player. As for the actual technique, let's keep it discussed in asciinema/discussions#8.

Anyway, what we can add to v2 header is the optional key named colors (or theme ?):

{
  "version": 2,
  "width": 80,
  "height": 24,
  "theme": {
    "fg": "#d0d0d0",
    "bg": "#212121",
    "palette": "#151515:#ac4142:#7e8e50:#e5b567:#6c99bb:#9f4e85:#7dd6cf:#d0d0d0:#505050:#ac4142:#7e8e50:#e5b567:#6c99bb:#9f4e85:#7dd6cf:#f5f5f5"
  }
}

I took inspiration to use single string value for all 16 colors from Terminator theme syntax which is compact and it's easy to write/read/parse for both humans and machines.

[ku1ik]

This was added to the v2 spec in #233.