WL#705: Improve mysqldumpslow to run explain

One of the most typical tasks in MySQL optimization it is analyzing slow query
log by running EXPLAIN statements to the queries.

It is good if this functionality is added to mysqldumpslow.

To avoid exessive explain output it would be good to have an option limiting
explain call to the queries examined more/less than N rows or taking more/less
than some time. This would add support for "optimize slowest queries first"