feature(sequence-table) adding columnNames callback signature
feature(sequence-table) adding `columnGroups` mapping
feature(sequence-table) adding `data-group="…"` mapping to data cells
feature(sequence-table) classify columnGroups row
feature(sequence-table-body) adding `titleCell` callback
feature(sequence-table) column-spanning of group should be handed manually
bugfix(sequence-table) treat options as optional
feature(demo) make demo interactive and add columnGroups
version bump to 0.2.0