feat(material-experimental/theming): checkbox theme customization

[wagnermaciel]

Example usage:

html {
    @include mat.all-component-themes($light-theme, (
        checkbox: (
            selected-checkmark-color: blue,
        )
    ));
}

Alternate usage:

html {
    @include mat.checkbox-theme($light-theme, (
      selected-checkmark-color: blue,
    ));
}

Notes about the API:

• An error is thrown if a user passes in a token that does not exist in either the m2/mat or m2/mdc.
• Prefixes are determined automagically for the user based on whether the token exists in the m2/mat or m2/mdc tokens.

[crisbeto]

IMO we shouldn't add tokens to the existing mixins. We should just one extra mixin like customize (name to be debated) where users pass in a flat map of tokens.