Fix names of CellSelection.colSelection and CellSelection.rowSelection
Fix names of CellSelection.colSelection and CellSelection.rowSelection

BREAKING: The names of `CellSelection.colSelection` and
`CellSelection.rowSelection` were swapped (they now actually describe
what the methods do).

Issue #44

ProseMirror/prosemirror-tables,ProseMirror/prosemirror-tables,ProseMirror/prosemirror-tables