Add RichTextBlockFactory

[estyxx]

I know there was #25 PR but was super super old, there were too much conflicts to solve :(

[jams2]

Hi @estyxx - are you still up for getting this over the line?

I think we need the following changes:

• the factory should take either a string or a RichText object, and always generate a RichText value; and
• we should have tests.

Please let me know if you don't have the opportunity to keep working on this.