Allow to serve a full directory of ZIM files

[kelson42]

Today kiwix-serve can serve:

• A list of ZIM files
• An XML library

There is no way to just give a directory and kiwix-serve will explore the library and load all the ZIM files.

To some extend, we can do that with globbing, but this is a kind of a workaround to the problem.

We should just allow to give a directory as argument and then kiwix-serve would visit the whole directory arborescence for ZIM files and load them. Any ZIM file which is not loadable (for example because not readable) should generate an error (like we already do today).

This issue is pretty much connected to #740

This issue is a follow-up of #735