cylc poll-all: no options given then poll all

Closes #1177

Tests added.

Polls all tasks when no options are given at the command line.

Note that in my environment including on master tests/cylc-show/03-clock-triggered-non-utc-mode.t fails.
I think we can improve the test by adding:
* some idle tasks.
* some submitted tasks.
* perhaps more running tasks?

N.B. Submitted tasks can be created by using `[job submission]method=at` with something like `[job submission]command template = at now + 5 minutes`. The job will be submitted to `at` but will not running for the next 5 minutes.

Do we need to test this new functionality with a cycling suite as well?