[Feature] Quote on selection, like the behavior of GitHub comments

[ofey404]

Now the "quote" button would capture the whole reply, which makes it not so useful.

It would be nice if the "quote" button captures only selected text if there is a selection.

This snapshot illustrates this idea:

And when there is no selection, the "quote" button can fall back to default behavior.

Actually, the GitHub "quote reply" works this way.

[Bin-Huang]

Thank you for your suggestion. I need to take a look at how to implement it first.