Make react markdown (renderer) more configurable

[elboletaire]

There are multiple possible solutions:

1. Allow to pass a configuration object that would be spread to ReactMarkdown component. This should allow reconfiguring all the defined components.
2. Additionally to be using a component to render the markdown, a simple function with the raw text could be provided, giving to the end-developer the possibility to use any renderer, and completely removing ReactMarkdown as a dependency (and would close Make react-markdown optional #5). This has a major drawback tho: it's breaking with the current behavior, and would require changes not only in ui-scaffold, but also in ui-explorer.
3. Create a markdown context and allow to configure the Markdown component when defining such context.

I think it'll be 1, but it could be a fusion between 1 and 2 too (meaning, adding the render function logic, but maintaining the current default behavior)