Add support for functional components

There might be a bit of trickery needed to make this happen, but currently a `.md` file like this:

```
# Example

import ExampleImport from '@/components/ExampleImport.vue';

export default {
functional: true,
components: {
ExampleImport,
},
};

```

Will throw `[Vue warn]: Error in render: "TypeError: _vm is undefined"`. This also happens when in `.vue` files, you define both a `script` and a `template` tag, instead of either/or. Would it be possible to see if someone has set `functional: true` on their component and inlining the template if so? This could provide quite a significant speed-up in rendering stateless components.

Start by reproducing the reported TypeError with the `.md` example and with a `.vue` file containing both `script` and `template` tags. Trace how markdown and Vue components are transformed, then verify that components declaring `functional: true` render without the `_vm` error and retain their imports and templates.

Written by the indexing model from the issue text.