Support zipped mods

Add support for zipped archives under our VFS, this would reduce the amount of open file descriptors and potentially increase performance while simplifying the whole modding process. It's reasonable to assume that we are backwards compatible anyway as there is no current 'standard' when it comes to Bethesda's games in terms of mods aside from esm/esp + bsa (or loose files), so defining a standard for OpenMW would also benefit the community anyway. Modders can always extract the archive for Morrowind.exe like they normally would.

Define a layout/structure of the contents inside the archive so that OpenMW can trivially add the archive to its VFS.

Example:

/readme.txt  
/license.txt  
/mod.esp  
/mod_optional.omwaddon  
/Meshes/grass/Grass_AC_1.nif  
/Textures/grass/TX_grass_AC.dds  
/Scripts/mod.lua  

Notes:

• This leaves room later for other tools that would re-pack mods that aren't packed in this way to make them OpenMW compatible.
• I see no need to add "/Data Files/" to this, but I'm not against it either. I see it as useless as "Data Files" is more or less our VFS root's directory anyway.

Follow-up discussions:

• How to support this in the launcher
• Support for complex mods like those using BAIN
• Support other compression/container formats?