epic: `shallow: true` mounting option (previously `shallowMount`)

[lmiller1990]

Will probably rely on #9

Instead of shallowMount, I think we should have a shallow mounting option (discussed internally).

const Comp = mount({
  shallow: true
})

How this should work

shallowMount was pretty buggy in VTU beta. It should completely stub out and child components. So:

<template>
  <div>Content</div>
  <Foo />
  <Bar>
    <div>BAR</div>
  </Bar>
</template>

Should render as follows when using shallow:

<div>Content</div>
<foo-stub />
<bar-stub />

Whether it's <FooStub /> or <foo-stub> /> doesn't matter, the point is that the components are no longer rendered, nor is any of their code executed.

Things to consider:

• should a user be able to find a stubbed out component? Feels a bit weird to try and find a component that is purposefully NOT rendered. But it might be a common use case
• current find does not support find(Component) syntax. I'm not sure it should - it was impossible find. One such issue: Some stubbed Vuetify components can't be found with find wrapper method vuejs/vue-test-utils#1272 and
• consider this use case Can't trigger event of child component vuejs/vue-test-utils#1410 people often stub out components then want to emit an event from them. Can we handle this better?