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.

Rework psuedoelement infrastructure so they can be their own boxes.

Test/fix parsing of psuedoclasses/elements.

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

Add support for psuedoelements/classes.

Tidyup testsuite code, according to newly enabled compiler warnings.

Code cleanliness fixes.

Improve code cleanliness to avoid future bugs.

Data.CSS.Style.Selector.Index: fix potential crash.

Code cleanliness: Interpret Data.CSS.Style.Importance.splitProperties return value correctly.

Factorize out cascade logic.

Add method to override CSS inheritance.

Parse HTML 'style' attributes.

Have Stylish HTML Conduit depend on a non-buggy version of Stylish Haskell.

Fix parser freezes, blocking issue for Rhapsode.

Fix incorrect handling of invalid syntaxes.

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.