Export traversePrepopulatedStyles

Add additional hooks for callers:

* Resolve relative URLs.
* Set priority on any stylesheet implementation.
* Substitute content during XML Conduit traversal in place of computed styles.

Update Stylish HTML Conduit to use fixed psuedoelement handling.

Add support for ::before/::after psuedoclasses to HTML Conduit bindings.

Parse HTML 'style' attributes.

Improve traverseStyles type signature.

Allow differing output types and give text nodes style if they want it.

Improve concision of Stylish HTML Conduit's API.

Draft html-conduit to Stylish Haskell bindings.

Initialize stylish-html-conduit sidemodule.