Add feature to link to, rather than browse, directories with their own i...

This adds a useful new feature.  The user can specify in their config some filenames (defaults provided, like `index.htm`, `index.php`, etc) that, if detected in a directory, will result in that directory not being browsed but rather linked directly.

The use case that prompted this was having DirectoryLister drive a quick developer portal: we had directories with binaries, etc. to download, but also uploaded Doxygen-generated documentation in the same place and didn't want to have a manually-updated homepage.