Configure widget via data-waterfall attributes

To override the default configuration of the widget, specify a
`data-waterfall-foo` attribute, where 'foo' is the configuration property to
change.

The widget currently supports the following attributes (prefaced by
`data-waterfall-`:
- subscribe-frame-url: URL of the iframe subscription server
- class-prefix: CSS class prefix to use when rendering the widget
- mobile-number-legend: text to display for the mobile number fieldset
- area-code-label: text labeling the area code field
- prefix-label: text labeling the prefix field
- line-no-label: text labeling the line number field
- sign-up-button-label: submit button text
- rates-warning: markup to render warning of data and message charges
- terms: markup to render laying out terms and conditions

See `src/js/waterfall-subscription-widget/default-content.js` for the defaults.