feat: add theme

[GzhiYi]

Hi!
I think it's not well enough when i try to change the popover color.
I found this issue and make some improvements.

Now, you can change the main color of the popover:

template(lang="pug")
  v-tour(name='myTour' :steps='steps' :options="tourOptions")
// ...

data() {
  return {
    tourOptions: {
      theme: '#506eff'
    }
  }
}