Add support for Excel / OO Calc import

Low-priority wishlist item, since anyone who's got a spreadsheet can export a CSV and use the existing tabular importer. Apache POI supports Excel; for ODS there seem to be various options: <http://www.jopendocument.org/>, <https://odftoolkit.org/>, <https://github.com/miachm/SODS>, <https://github.com/jferard/fastods>, more listed at <http://opendocumentformat.org/developers/>.