Implement Placeholder Text for MdInput as per material.io guideline

[borislemke]

Bug, feature request, or proposal:

Feature Request

What is the expected behavior?

Able to add hint / helper text alongside label

What is the current behavior?

It is not possible to have a label AND helper text (or actual input placeholder) together.

What are the steps to reproduce?

API for this feature does not currently exist

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

Follow official material guideline

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

All

Is there anything else we should know?

[mmalerba]

closing as duplicate of #6194

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