Support multiple files

[rmariano]

Ability to compress multiple files, packaging the compression into a single one.

This changes the cli interface, for now the user has to specify the name of the output file first (default one will probably not work anymore), and then the list of the files to search (similar to tar, etc.), like:

pycompress --ouptut <ofilename> [files...]

The compression can be done sequentially, no need to parallelism; Any sort of optimisation will be done later on.