Include md-optgroup label in md-select selected item

[larjohn]

Bug, feature request, or proposal:

Feature request

What is the expected behavior?

When md-select has some md-optgroup's there might be some options that have the same label. For instance:

Cars
|

--- SUV
--------Diesel
--------Petrol
--- Mini
----
--------Diesel
--------Petrol

It should be nice if the selected value (when the dropdown is closed) also included the option group label, like SUV-->Diesel

What is the current behavior?

Only the option label is shown

What are the steps to reproduce?

Use the md-select with some md-optgroup's

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

I know this is the default select behavior, but I might see a usability improvement here.

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

All

Is there anything else we should know?

[larjohn]

Related to #2275

[jelbourn]

The select trigger label can be fully customized as of #3341

[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.}