support md-input readonly attribute

[ribizli]

Note: for support questions, please use one of these channels:
https://github.com/angular/material2/blob/master/CONTRIBUTING.md#question.
This repository's issues are reserved for feature requests and bug reports.

• Do you want to request a feature or report a bug?
  feature
• What is the current behavior?
  attribute readonly is not supported and not transfered to the wrapped input element
• If the current behavior is a bug,
  please provide steps to reproduce and if possible a minimal demo of the problem
  via https://plnkr.co or similar.
• What is the expected behavior?
  support readonly attribute
• What is the motivation / use case for changing the behavior?
  one might need this feature from html input element
• Which version of Angular and Material, and which browser and OS does this issue affect?
  Did this work in previous versions of Angular / Material?
  Please also test with the latest stable and snapshot versions.
  all, feature not implemented
• Other information
  (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix)

[alexandrejunges]

Is anybody working on a PR for this?
I want to start helping with AM2 development and I think this issue would be a nice start.

[matthewdenobrega]

I've taken a stab at fairly comprehensive input attribute support in #343.

[jelbourn]

Closed via #447

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