feat(select): add ability to customize the select trigger

[crisbeto]

Adds the md-select-trigger directive which allows users to customize the selected value. E.g. it is now possible to do something like this, if the user wanted to reverse the selected label for some reason:

<md-select placeholder="Food" [formControl]="control" #select="mdSelect">
  <md-select-trigger>
    {{ select.selected?.viewValue.split('').reverse().join('') }}
  </md-select-trigger>
  <md-option *ngFor="let food of foods" [value]="food.value">
    {{ food.viewValue }}
  </md-option>
</md-select>

Fixes #2275.

[fxck]

Any progress on this? I think it could be useful for #2722 (comment)

[fxck]

How's this feature coming? ;)

[jelbourn]

LGTM on API / approach

Not sure how this will affect @mmalerba's work

[mmalerba]

Is this the equivalent of what md-placeholder is for input? I plan to refactor md-select to work inside md-input-container (which is currently being renamed md-form-field in #6331 ). Once that is done the select can just take advantage of all the placeholder/hint/error features currently available on the input

[crisbeto]

Pretty much, It was quite a while ago since I opened this PR.

[fxck]

mm?

[crisbeto]

Hmm looking through it again, this won't be the same as the md-placeholder @mmalerba. This allows for the displayed value to be overwritten, not the placeholder. It should probably be called md-select-value though. I'll reopen and rebase it.

[mmalerba]

@jelbourn does this new name sound good to you?

[crisbeto]

We agreed on it in Slack @mmalerba.

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