Icons are not vertically aligned inside mat-option elements

[benelliott]

Bug, feature request, or proposal:

mat-icon instances are not vertically aligned with the option text when placed inside mat-option elements.

What is the expected behavior?

Icons should be vertically aligned.

What is the current behavior?

Not vertically aligned.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-5num53

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

Aesthetics, plus consistency with other APIs like mat-menu-item in which icons are vertically aligned by default.

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

Latest StackBlitz template, Chrome 64, Win 10

Is there anything else we should know?

Maybe vertical-align: middle should be added to mat-menu-item-base here and removed here.

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