Rowspan support on sortable tables

[zachleat]

Currently rowspan is not supported with the sortable plugin, as documented on https://github.com/filamentgroup/tablesaw#limitations

Related: #247

[zachleat]

This repository is now using lodash style issue management for enhancements. This means enhancement issues will now be closed instead of leaving them open.

The enhancement backlog can be found here: https://github.com/filamentgroup/tablesaw/issues?utf8=%E2%9C%93&q=label%3Aneeds-votes+sort%3Areactions-%2B1-desc+

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