Allow filtering on aggregates.

Kinda like SQL's HAVING clause but deduced from what's involved in --field expression rather than a separate operator. Eg count(exception!=)>2. Show me things with at least two exceptions.
Add --live-ascii option.
Move e.toSeq() call into Tabular.
Yak shave: de-duplication.
Support calc(max(time)-min(time)).
Filter on calc()>N
Sort by a calc() field
Avoid umpteen matches on SubtractTimeExpr()
Support calc(a/b).
Try to make mutable builders more obvious.
No more need for minStr() and maxStr().

Regular min() and max() try numerical compare and fall back.
Allow -f delay>N when -t doesn't include delay.