Add Outline Buttons

Bootstrap provides outline versions of all its button classes.

Examples per the documentation:

<button type="button" class="btn btn-outline-primary">Primary</button>
<button type="button" class="btn btn-outline-secondary">Secondary</button>
<button type="button" class="btn btn-outline-success">Success</button>
<button type="button" class="btn btn-outline-info">Info</button>
<button type="button" class="btn btn-outline-warning">Warning</button>
<button type="button" class="btn btn-outline-danger">Danger</button>

I propose we add a checkbox that says "Use Outline Style" underneath the style selector to accommodate this.

Patch forthcoming.