feat(primeng): datepicker additional props

[frixou89]

Additional props for PrimeNG datepicker

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature: Adds missing properties for PrimeNG datepicker.

What is the current behavior? (You can also link to an open issue here)
Datepicker field type for PrimeNG is missing the following options

  showSeconds?: boolean;
  showClear?: boolean;
  stepHour?: number;
  stepMinute?: number;
  stepSecond?: number;

What is the new behavior (if this is a feature change)?
Added support for the missing properties

Please check if the PR fulfills these requirements

• The commit messages follow our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• A unit test has been written for this change.
• Running npm run build produced a successful build. (Unit testing can be done by running npm test;)
• My code has been linted. (npm run lint to do this. npm run build will fail if there are files not linted.)

Other information:
Also updated demo to display an example with datetime picker

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