ENH: summary as_text option for separator lines

[josef-pkt]

The VECM summary creates 10 tables. When they are combined to string, then the tables look crowded because there is no separator line between tables.

[josef-pkt]

ss = '\n\n'.join(str(table) for table in summ.tables)
print(ss)

works

However, the automatic html tables in the notebook look better structure for VECM, separate html table with title or preceding descriptive line in different style.