Implement 24-bit web color format, like #ffaabb

Actually modern terminals support 24bit colors using the following sequences.
* `38;2;<r>;<g>;<b>` - foreground
* `48;2;<r>;<g>;<b>` - background

I'm net sure yet if it's worth to support the web format, like `#ffaabb`. Because then you need to come with a way to encode foreground and background.