Public API for vScrollMargin

[mcolyer]

This was originally brought up as a part of atom/vim-mode#77, currently the amount of vertical scroll padding isn't configurable and it isn't queryable by extensions. It would atleast be nice if plugins that were interested in this could access the value.

Ref:
https://github.com/atom/atom/blob/master/src/editor-view.coffee#L63

What are everyone's thoughts here?

[nathansobo]

Sounds like a good idea. Reading it should be no problem. Assigning it could be a little trickier for cases where the cursor is inside the newly assigned margin.

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