Add support for sticky file headers by adding a `stickyFileHeaders`
option to the `Diff2HtmlUI`. By default this feature is disabled. Also
document this option in the README.
The feature is implemented through an optional CSS class on top of the
pre-existing `.d2h-file-header` class. The new class is added on all
file headers if the option is set to `true` (or the `stickyFileHeaders`
method is called).
This class, `.d2h-sticky-header`, has the minimum amount of styling to
get the desired effect. The `position` and `top` values make the headers
stick to the top as long as the wrapper is in the view. The `z-index`
value is needed to ensure the header is displayed over all other content
in the wrapper. In particular, from my testing in Firefox (106.0.2), the
line numbers would display over the header if the `z-index` value isn't
set.
