Alow more task summary info as event handler args.

[hjoliver]

Following on from #2441 - allow remaining task summary data as event handler args: submit_time, start_time, finish_time, user@host. This lets us use event handler scripts for routine reporting, at least until we hash out the details of #2442

@cylc/core do you see any reason not to do this, before I go ahead with documentation and extending the tests?

[matthewrmshin]

Looks like a good move. (One day, we'll get TaskProxy.summary set up correctly so we don't have to build another dict!)

[hjoliver]

Existing test extended to cover the new args.

[hjoliver]

Rebased to bring in the pep8 -> pycodestyle change.