doc: add cell.merge feature analysis
acpt: add cel-merge.feature
test: add tests for cell.merge, cell.row_index and cell.column_index
oxml: add CT_TcPr/CT_Tc gridSpan support
oxml: add CT_TcPr/CT_Tc vMerge support
oxml: add CT_TcPr/CT_Tc hMerge support
tbl: add cell.merge(), cell.row_index and cell.column_index