Add support for outline summary direction

[mlh758]

Description

By default row outline summaries (the little + box next to the row) appear above the row. There is a sheet setting to flip these below. In Ruby Axlsx this is done by: sheet.sheet_pr.outline_pr.summary_below = false. It would be nice to have the same feature here.

It seems like it would need to go here but I'm not sure how it would be serialized after adding it.

[mlh758]

It looks like serialization would happen here but I'll need to figure out what to actually call the element. Or trial and error it.

I would be willing to PR this change in once I figure it out.

Edit: For my own reference, the spec can be found on msdn

[mlh758]

It should end up looking like this:

<sheetPr >
    <outlinePr summaryBelow="0" />
    <pageSetUpPr fitToPage="0" />
...
</sheetPr>

I've got part of an implementation together but it doesn't seem to be saving the attribute within outlinePr.