Adding --history-api-fallback CLI option to ng serve

[eib]

Fixes #9640 by adding a CLI option (defaults to current behavior).

While it would be swell to pull the correct behavior from each app's router configuration, that's a little outside my area of expertise. This doesn't feel important enough to polluting the .angular-cli.json schema over, either. In the end, just a CLI option is enough for my needs.

Feedback/edits welcome.

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[filipesilva]

Hi, we aren't thinking of adding new flags right now, and we are adding an extensibility model in the next major version that will allow you to replace the dev server. For these reasons I am closing this PR.

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