Enable passing html blocks into button

Useful if you want to include a spinner for the pending state.

Syntax:
```
{{#async-button action="save" class="html"}}
  {{#button-state name='default'}}
    <strong>Save</strong>
  {{/button-state}}
  {{#button-state name='pending'}}
    <strong>Saving...</strong>
  {{/button-state}}
  {{#button-state name='fulfilled'}}
    <strong>Saved!</strong>
  {{/button-state}}
  {{#button-state name='rejected'}}
    <strong>Fail!</strong>
  {{/button-state}}
{{/async-button}}
```
This is not a valid usecase as this can be accomplished already via pure CSS. I would prefer the README just make a note that the component can take a block form.