Support 256 Color

### Describe the feature you'd like

Support for [256 color](https://github.com/dracula/spec/issues/3) like the [vim plugin](https://github.com/dracula/vim/blob/master/autoload/dracula.vim)

### Describe risks you've considered

I can't think of any technical risks. It's possible some users could settle for 256 color config, if they can't easily figure out [how to get true color working](https://gist.github.com/andersevenrud/015e61af2fd264371032763d4ed965b6).

### Describe alternatives you've considered

My issue is that [Mosh](https://mosh.org) doesn't support true color. I discovered [a patch](https://github.com/mobile-shell/mosh/pull/939) which should work for me, but it appears to [not work correctly in some uses cases](https://github.com/mobile-shell/mosh/pull/939#issuecomment-340127135).