bug: can't add button attributes dynamically

[brandyscarney]

We fixed color by adding the color attribute, but if they use any other button attribute it won't add the class and won't get the proper styling.

<button [attr.outline]="slide.title ? '' : null">test</button>

[adamdbradley]

These are now possible to dynamically add and remove button attributes.

<button outline>
<button outline="true">
<button outline="false">
<button [outline]="true">
<button [outline]="false">