adding a "random" theme.

hi.
I'm not sure which theme to choose,
and I know oh-my-zsh has this thing called "random" theme.
I didn't take the code from there, just the idea.

Why wouldn't you just source the new theme?

```bash
  source "$BASH_IT/themes/$BASH_IT_THEME/$BASH_IT_THEME.theme.bash"
```
It's probably only like .1 milliseconds but it doesn't make sense to source `appearance.bash` again. Also, future changes may break as you're probably not expecting it to run twice.