feat(mat-select): add "select other" optional functionality

#### Bug, feature request, or proposal:
Feature request

#### What is the expected behavior?
Add "select other" / "select custom" *optional* functionality to `mat-select`. This would provide a seamless way for a dev to let a user select from a predefined list of drop down items _**or**_ to provide a custom selection. I imagine if someone chose the "other" option of a `mat-select` dropdown, a `matInput` text field would appear and accept input. Crucially, user updates to this "other" `matInput` field would change the value of the `FormControl` associated with the `mat-select` field.

#### What is the current behavior?

Currently, this functionality can already be achieved however it is not easy. In my app I have made a custom "select-other" form control by combining a `mat-select` and `matInput` controls, both of which have their own `FormControl`. The component needs to flatten output from both of these controls so that I can interact with the component by just giving it one `FormControl` as input. It works, but it's not super clean. I'm assuming that a "select other" option (similar to a Google Form's "other" radio button option) is a pretty common design need.

#### What is the use-case or motivation for changing an existing behavior?
Increase developer happiness.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
5.x

#### Is there anything else we should know?

Start by reading the Angular Material mat-select and matInput entry points, then review how their associated FormControls currently expose values. Define how an optional "other" choice and custom text should interact with mat-select and the FormControl, and establish tests for predefined and custom selections before implementation.

Written by the indexing model from the issue text.