Allow to customize when md-error is shown

Bug, feature request, or proposal:

Feature request

What is the expected behavior?

I would like to be able to customize when the md-error directive is visible. E.g. when ctrl.dirty && ctrl.invalid.

What is the current behavior?

As per #3560 my understanding is that the md-error is only shown when the control is: (isInvalid && (isTouched || isSubmitted))

What are the steps to reproduce?

See my Plunker.

• Name1 is using md-error. Error will only show after control been touched.
• Name2 is using an old workaround with md-hint. It can show error directly when dirty.

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

For 99% of the time I believe the current implementation is the best behavior.

However, for special cases, you will need to be able to modify when errors are shown. For example instantly as the example from the Angular documentation.

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

Angular v4.0.3 Material v2.0.0-beta.3

Is there anything else we should know?

I assume my request is not currently possible as my question on StackOverflow did not get any suggested answers.

[HopScotch47]

Instead of configuring the whole behaviour to trigger a special case, you could simply do this:

const ctrl = this.form.controls['ctrlName'];
ctrl.markAsTouched();

for the control in question. This at least is what I do in such cases.

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