Feature Request: Allow custom coerceDateProperty strategy

[paulussup]

https://github.com/angular/material2/blob/d2ceb2c755e20252f02fda11dc4eef55d20f4bbe/src/lib/datepicker/datepicker-input.ts#L37

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Ability to inject custom date coercion function or have numbers coerced as Unix Timestamps.

What is the current behavior?

Currrently strings are parsed as Iso8601 https://github.com/angular/material2/blob/d2ceb2c755e20252f02fda11dc4eef55d20f4bbe/src/lib/datepicker/coerce-date-property.ts#L23
and type == 'number' throw errors

https://github.com/angular/material2/blob/d2ceb2c755e20252f02fda11dc4eef55d20f4bbe/src/lib/datepicker/coerce-date-property.ts#L33

What are the steps to reproduce?

plunkr

What is the use-case or motivation for changing an existing behavior?

Numbers should be coerced as Unix Timestamps.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Is there anything else we should know?

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