Core:
- Added `duplicateSpan` option (default is `true`).
- Renamed `$.tablesorter.formatSortingOrder` to `$.tablesorter.getOrder`.
- Include `table` in console.error if an issue is encountered during
initialization.
- Clean up warning when no parser is found for given data.
- Fix `config.sortVars` js error for non-existent header cells.
- Added unit tests.
- Added "example-colspan.html" demo.
Filter:
- Filters that span multiple columns now have the correct data-column set.
- Consolidated code that parsed data-column ranges into `findRange` function.
- Added unit tests
