- Update documentation
- Implement real character set conversion (using iconv); there should be a
  command-line option to set output charset and another to override input
  charset. Default: auto-detection for both.
- Define an XML DTD and stick to it. Look into the OpenOffice spreadsheet
  format - we don't need two different spreadsheet XML formats...
- Modularize code. Separate XLS reading into a library, make a frontend with
  various output formats: XML, HTML, CSV...
- Make main_line_processor()'s working buffer grow dynamically
- Notes from Steve Grubb:
  - Support String formulas
  - Reduce memory footprint
  - Support data types by using their format string
