config: move flake8 targets to tox.ini
fix: correct error message template
doc: add cell.merge feature analysis
acpt: add scenarios for cell access

* refactor tbl-item-access to remove cell access scenarios
tbl: add _Cell.text getter
tbl: reimplement _Row.cells

Temporarily named _Row.cells_new
tbl: add _Row.table
tbl: add _Rows.table
tbl: add Table.table
tbl: add _Row._index
tbl: add Table.row_cells()
tbl: add Table._column_count
test: add snippet infrastructure
tbl: add Table._cells
tbl: remove now-dead code

* remove _RowCells and tests
* make _Row.cells_new _Row.cells and remove prior version
acpt: fix paste-repeated typo in several features
tbl: reimplement _Column.cells
tbl: add _Column.table
tbl: add _Columns.table
tbl: add _Column._index