Feature: Open preview window in same cursor line

[ellisonleao]

From issue #26

[RaafatTurki]

This would go hand-in-hand with #78

[RaafatTurki]

This can be achieved by rendering the markdown file from start to cursor line in a scratch buffer.
The last line number in that buffer is the line we should focus on in the actual :Glow window.

[mkalpakis]

I would also be interested in this, I am editing a long markdown file and having to scroll all the way down each time is frustrating.