The gatsby lumen starter currently supports a `socialImage` field in markdown content that will set the `twitter:image` and `og:image` meta tags in the layout component. This PR adds an "Social Image" widget that allows users to set the socialImage field through the netlifycms admin page.
preview_path allows for UI linkage between the Netlify CMS UI
and the associated preview deploy build. Also adds a commented
out publish_mode configuration to enable the UI authoring flow,
which continues to default to publish direct to master.
