Add readOnly Input property to md-input

[Angelmmiguel]

Bug, feature request, or proposal:

Feature request / Bug

What is the expected behavior?

Add readOnly or a similar property to md-input must add the readonly attribute to the rendered input tag.

What is the current behavior?

We can't set a md-input as read-only

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

For example, I'm adding an input with a copy to clipboard functionality. You can see an example in
Rubygems. Also, this is a basic functionality of HTML input tags.

[Angelmmiguel]

Sorry for opening this issue. I found the same one, so I'm going to close this as duplicated by #343

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}