ngLib(feat) manual Ready signaling

[SanderElias]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

No way to manually trigger "idle"

Issue Number: N/A

What is the new behavior?

A way to set manual idle triggering globally using the forRoot,
and a way to set it per Route, using scullyconfig like this:

    '/manualIdle': {
      type: 'default',
      manualIdleCheck: true,
    },

With this setting, the developer becomes responsible for triggering IdleMonitorService.fireManualMyAppReadyEvent() when the app is ready for rendering.

closes #345

Does this PR introduce a breaking change?

• Yes
• No

Other information

[jorgeucano]

LGTM