Tksheet

Changed:
- Treeview mode slight appearance change

Added:
- Treeview mode documentation

Added:
- New option to address [247](https://github.com/ragardner/tksheet/issues/247)

Added:
- New options to address [246](https://github.com/ragardner/tksheet/issues/246)

Fixed:
- Redundant code causing potential redraw error
- Wrong version number in `__init__.py`

Fixed:
- [245](https://github.com/ragardner/tksheet/issues/245)

Changed:
- Moved variable `default_index` to sheet options as `default_row_index`
- Moved variable `default_header` to sheet options

Improved:
- Documentation

Pull Requests:
- [237](https://github.com/ragardner/tksheet/pull/237)