feat(select): Add select header to the md-select

Bug, feature request, or proposal:

Feature request

Yesterday I filled a pull request ( #2797 ) for adding a search bar in the md-select component. As @jelbourn pointed out this wasn’t the right way to add this feature, so now I’m willing to do it the right way. What’s the best way to address this? Just implement the same functionality as material does it?

That will mean something like this is possible:

<md-select placeholder="Favorite food">
    <md-select-header>
        <input [ngModel]="searchTerm" type="search" placeholder="Search for a vegetable..">
    </md-select-header>   
        <md-option *ngFor="let food of foods | filter " [value]="food.value">
            {{ food.viewValue }}
        </md-option>
</md-select>

[crisbeto]

There’s a pending pull request at https://github.com/angular/material2/pull/7835.

[liesahead]

Still no? More than 1 year passed