Fixed a:link and visited(), added .visitedLink

* The CSS selector ":link" in stylesheets is now converted to
[class*=Link] at runtime. This is to provide backwards-compatibility for
people who used "a:link" to refer to Twine links (which is no longer
usable as internal links no longer have href attributes). It's a mean
sight of a hack, but so be it - I shan't be adding back the
"javascript:void(0)" href no matter what you say.
* Added the "visitedLink" class, which is applied to passage links that
link to already visited passages. ":visited" is converted to
".visitedLink" much like the above. This provides a surprising deal of
power - you can give .visitedLink "display:none", for instance.
* The StorySettings passage is no longer converted to all-lowercase at
build time. This should make HTML exporting/importing more symmetrical.