Programmatic Selection support

[m1guelpf]

I'm trying to add inline commenting to a markdown document (where you first select some text and then write a comment, which shows up either inline or to the side) similar to Google Docs.

While Textual supports selecting text, it doesn't currently allow you to retrieve the value of the selected text or relate the selected range to the original markdown document, nor does it allow for dynamically adding a highlight once a comment has been placed.