Add option for # of lines to scroll

[david-hollifield]

First. Bravo! I've been looking for a VS/Surface solution for ages. This extension is GREAT! Thank you!

Installed product versions

• Visual Studio: [2017 Community v15.9.3]
• This extension: [0.8.68]

Description

Is there any way to change the number of lines to scroll when in Scroll Mode?

Steps to recreate

None

Current behavior

Only a single line is scrolled which is pretty slow

Expected behavior

Scroll the number of lines that are configured.

[philipwolfe]

This was added in #36. I intended on a separate PR but it sneaked in. I was going to add a reference to this on that PR. Oh well.