Date pipe: add skeleton formats

[JohnKis]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

Date pipe doesn't offer a way to use custom date formats that respect the ordering of the date components according to the locale.

Expected behavior

There should be a way to specify a custom format while keeping ordering of the locale. In #11661 it was suggested that this should be achievable by adding support for CLDR skeletons (see "Additional Date-Time Formats" in http://cldr.unicode.org/translation/date-time-patterns)

The following example illustrates the expected output of the pipe with and without respecting the ordering of the locale:

// LOCALE_ID: 'en-GB'
{{ date | date:'EEE d MMM' }}
// Current: Tue 20 Oct
// Locale ordering: Tue 20 Oct

// LOCALE_ID: 'en-US'
{{ date | date:'EEE d MMM' }}
// Current: Tue 20 Oct
// Locale ordering: Tue, Oct 20

// LOCALE_ID: 'hu-HU'
{{ date | date:'EEE d MMM' }}
// Current: K 20 okt.
// Locale ordering: okt. 20., K

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

I believe it would be a useful feature for internationalised apps. For instance, I have multiple apps that render only specific date components that cannot be expressed using a standard format.

Environment

Angular version: 4.1.3
It is still the case in the most recent Angular version
Browser:
- [x] Chrome (desktop) version XX
- [x] Chrome (Android) version XX
- [x] Chrome (iOS) version XX
- [x] Firefox version XX
- [x] Safari (desktop) version XX
- [x] Safari (iOS) version XX
- [x] IE version XX
- [x] Edge version XX

For Tooling issues:
- Node version: v6.3.0
- Platform: Mac

Others:

[ocombe]

To do that we may have to add the available formats to the extra locale data files: https://github.com/unicode-cldr/cldr-dates-full/blob/master/main/en/ca-generic.json#L333 and then follow this guide to use the correct format: http://www.unicode.org/reports/tr35/tr35-dates.html#availableFormats_appendItems

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[petebacondarwin]

It doesn't look like there is much interest in this feature, and it would increase the size of our local files and code in @angular/common to handle it.

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