rose date: support more letter options

* `--parse-format=FORMAT` can now be `-p FORMAT` (for `strptime`).
* `--format=FORMAT` is now a shorthand for `--print-format=FORMAT`
  (but not `--parse-format=FORMAT`).
* `--print-format=FORMAT` can now be `-f FORMAT` (for `strftime`).

Combine `--print-format=FORMAT` and `--format=FORMAT` internally. The
effect is that `rosie lookup` and `rosie ls` will now accept
`--print-format=FORMAT`.
Merge branch 'master' into rose-date-letter-opts
Merge pull request #799 from matthewrmshin/rose-date-letter-opts

rose date: support more letter options
Fixed a missing "-c" in clock-triggered tutorial.
Merge pull request #805 from arjclark/clock-triggered-typo

rose documentation: Fix a missing "-c" in clock-triggered tutorial.
Merge branch 'master' of github.com:metomi/rose into gh-pages