Feature request: Folder and multiple files support

[M-Gonzalo]

Premature optimisation is the root of all evils, so maybe this tiny 2-files library can serve us well to the purpose of granting precomp the ability of being a full archiver with POSIX support.

I specially like this part:

If you want to read or write from something other than a file, the mtar_t struct can be manually initialized with your own callback functions and a stream pointer.