generalized index files, changed STRIP_INDEX_HTML to STRIP_INDEXES

* Changes ``STRIP_INDEX_HTML`` to ``STRIP_INDEXES``
* If ``STRIP_INDEX_HTML`` is not set and ``STRIP_INDEXES`` is, use that setting
* add ``INDEX_FILE``, which defaults to ``index.html`` but may be set to ``index.php`` or ``default.html``

And, a bonus feature to address the confusion:
* If ``PRETTY_URLS`` is on and ``STRIP_INDEXES`` is not set, default it to ``True``

I tested this on my site and it works well, but due to the outstanding bug with tests failing on my machine (quite odd) was unable to add new automated tests for them.