Version 0.2.0

### Added
- New function: xlsx_get_last_column().
- New xlsx_sheet_t member: last_column, that won't have value (NULL),
unless xlsx_get_last_column() gets called at least
once.
- New private (static) function, helper of xlsx_get_last_column():
withdraw_alphabetic_chars().
- Several new test assertions for the new functionality.

### Changed
- README.md