`loadUrl` enhancements
- Make `options` parameter optional
- Allow partial overriding of instance options when calling `loadUrl` directly
- Make `requestOptions` optional
- Document `loadUrl` usage and provide examples
Previously, Pjax would blur (remove focus) from the active element
regardless of where it was on the page. This restricts that to
happen only if the active element is contained by one of the
elements represented by options.selectors, because only those are
affected by Pjax.
Fixes#4
