Extended the cat-log command to task logs

[hjoliver]

This user guide tutorial related change allows cylc log to print task as well as suite logs:

cylc log my.suite              # print suite event log
cylc log -o my.suite          # print suite stdout log
cylc log -e my.suite          # print suite stderr log
cylc log my.suite foo.1     # print foo.1 task job script
cylc log -o my.suite foo.1 # print foo.1 task stdout log
cylc log -e my.suite foo.1 # print foo.1 task stderr log

It also removes the task filtering option from the command - just use grep.

@arjclark et.al. - please review.