Load fallback archives listed in openmw.cfg at startup

It makes running the game require having ran the importer.

Differences from my [previous pull request](https://github.com/zinnschlag/openmw/pull/548):
 - Engine::setArchives(vector<string>& archives) changed to Engine::addArchive(const string& archive): the former forced to provide a definitive list of archives to load, the latter allow to add archives in several times
 - no more default BSA archive loaded

A problem however: the search for the archive is case sensitive.