Accept options when registering plugin to override defaults

It would reduce duplication and declutter templates if the plugin allowed overriding defaults:

```js
import Vue from 'vue'
import VueObserveVisibility from 'vue-observe-visibility'

Vue.use(VueObserveVisibility, {
once: true
})
```

If majority of my use cases require `once` to be set to true, then I can just do that, and if I need to set it to false occasionally, I can still do:

```js