`<mat-button-toggle>` are on/off toggles with the appearance of a button. These toggles can be
configured to behave as either radio-buttons or checkboxes. While they can be standalone, they are
typically part of a `mat-button-toggle-group`.


<!-- example(button-toggle-overview) -->

### Exclusive selection vs. multiple selection
By default, `mat-button-toggle-group` acts like a radio-button group- only one item can be selected.
In this mode, the `value` of the `mat-button-toggle-group` will reflect the value of the selected
button and `ngModel` is supported.

Adding the `multiple` attribute allows multiple items to be selected (checkbox behavior). In this
mode the values of the toggles are not used, the `mat-button-toggle-group` does not have a value, 
and `ngModel` is not supported.

### Appearance
By default, the appearance of `mat-button-toggle-group` and `mat-button-toggle` will follow the
latest Material Design guidelines. If you want to, you can switch back to the appearance that was
following the previous Material Design spec by using the `appearance` input. The `appearance` can
be configured globally using the `MAT_BUTTON_TOGGLE_DEFAULT_OPTIONS` injection token.

<!-- example(button-toggle-appearance) -->

### Accessibility
The button-toggles will present themselves as either checkboxes or radio-buttons based on the
presence of the `multiple` attribute.

For button toggles containing only icons, each button toggle should be given a meaningful label via
`aria-label` or `aria-labelledby`.

For button toggle groups, each group should be given a meaningful label via `aria-label` or
`aria-labelledby`.


### Orientation
The button-toggles can be rendered in a vertical orientation by adding the `vertical` attribute.