Provide custom remark-stringify options

**Is your feature request related to a problem? Please describe.**
The markdown output works perfectly. However, we are using prettier and eslint to ensure consistent formatting of the whole project, including Markdown. The formatting rules differ between prettier and netlify-cms, e.g. prettier prefers `-` for unordered lists while the [default setting in netlify-cms](https://github.com/netlify/netlify-cms/blob/master/packages/netlify-cms-widget-markdown/src/serializers/index.js#L112) is `*`.

**Describe the solution you'd like**
The best solution would probably be to offer an interface that I can use in my custom `cms.js` to pass [arbitrary options](https://github.com/syntax-tree/mdast-util-to-markdown#formatting-options) to remark-stringify.

**Describe alternatives you've considered**
I could probably change my prettierrc to use the netlify-cms default rules, but users might have a different preference for various reasons.

**Additional context**
- supported options: https://github.com/syntax-tree/mdast-util-to-markdown#formatting-options
- netlify-cms default options are defined in https://github.com/netlify/netlify-cms/blob/master/packages/netlify-cms-widget-markdown/src/serializers/index.js#L112
- prettier Markdown formatting rules: https://github.com/prettier/prettier/pull/2943

Start by reading packages/netlify-cms-widget-markdown/src/serializers/index.js and compare its default remark-stringify settings with the linked mdast-util-to-markdown options. Trace how a custom cms.js configuration reaches the Markdown serializer; done means supported arbitrary formatting options can be supplied there and affect generated Markdown without changing existing defaults.

Written by the indexing model from the issue text.