Get a report of conflicting narrative content in the target .docx

[jcscottiii]

Broken out from #13
In order make it easier to incorporate of manual downstream edits of the output .docx into the upstream YAML, YAML authors want a report of conflicts between the existing .docx content and content renderable from YAML.

Acceptance Criteria

• Given a .docx with content in fields that we would render to
  • AND the existing content conflicts with what we would render
  • WHEN we try to render with the --diff flag
  • THEN all the conflicts are reported
  • AND the target .docx is not modified

[afeld]

Related (maybe a separate story?):

• A warning is shown when the the YAML contains a single narrative but the docx has multiple sections, or vice versa (i.e. there's a mismatch in the formats between the provided data and the template)