Add support for overriding parts of the template

[menewman]

For example, allow for adding a custom header, a custom footer, a custom loading indicator, or custom in-cell markup. For in-cell markup, it's especially important to allow consumers to specify a {{#link-to}}{{/link-to}} within the cell so that the content can be clickable and point to some other route.

[menewman]

The in-cell markup part of this feature request was implemented in PR #13. Leaving this open in general since there still isn't a way to override the header/footer/loading indicator.