(implements #414) Add "no-unused-components" rule

[michalsnik]

This PR implements rule proposed in #414

[ota-meshi]

Looks good!

[micbra]

Great work! :)
When can I expect to be able to use this rule? I'm using 5.0.0-beta.2 and it doesn't include the no-unused-components.js file

[michalsnik]

I'll release it later today @micbra :)

[AnonymousArthur]

What about syntax like:
<template><component is="component_name"/></template>
when

const component_1 = import('./component_1.vue')
export default {
  components: {
    'component_name': component_1
  }
}

this is primarily used for dynamic components.
This currently can not be identified as used component.