Inline mode for markdown widget

**Is your feature request related to a problem? Please describe.**

If a field should allow markdown, but not block level content (headers, quotes, etc), there isn't a great way to facilitate that without locking down the widget. Primary use cases:

- fields like an author bio that should only include inline markdown characters
- editor components, Eg., definition lists support inline markdown

**Describe the solution you'd like**

Some sort of inline mode:
- toggle on via config
- toggle for preview output to not be wrapped in a paragraph (definition list previews require this)
- parser should ignore block level markdown tokens and treat them as literal characters
- UI
- editor components dropdown forced hidden
- block level buttons forced hidden
- inline buttons still subject to optional `buttons` config

**Describe alternatives you've considered**

Removing UI functionality for adding blocks, including hiding/disabling the rich text/markdown toggle. Downside to this is that blocks can still be read in, and will still be parsed as blocks.

**Additional context**

https://github.com/netlify/netlify-cms/pull/3300