Closes #217 (closed) Closes #251 (closed)
This PR doesn't break "pipe-ability" (a concern voiced in #161 (closed)). The sheet being "operated on" is still passed onwards for piping.
The difference is that when --write-all-sheets is present, each sheet of the XLS or XLSX is written to a file (as <basename>_0.csv, <basename>_1.csv), as a side-effect.