Add ability to autoflow a whole selection (multiple paragraphs).

I find `autoflow` extremely helpful when writing Markdown documents, but it was annoying having to hit the hotkey for every paragraph.

This PR adds a new command palette entry `Autoflow: Reflow Selection` that will reflow your current selection. My workflow is now to write a document, then when it's all finished, select all the paragraphs and run the Reflow Selection command.

I wasn't sure what keymapping to use, so I have omitted a default one for now.