version bump 0.7.0: Basic write support

- very basic XLSX / XLSM write support with roundtrip tests (XLSB stubs)
- reorganized source tree
- new XLSB range check ensures that A1 is not emitted for empty sheets
- SSF table emitted in output (consistent with js-xls)
- CLI supports writing

Backwards-incompatible changes:
o new Property aliases (see CORE_PROPS and EXT_PROPS)
o FILETIME custom properties parsed as JS Dates
o `xlsx2csv` -> `xlsx` (and `bin/xlsx{2csv,}.njs`)