Add allow_pickle keyword to control pickle usage

[KennyChenBasis]

Fixes #50

I have kept the current defaults, though, again, we might want to change the defaults (if we were to change the defaults to match numpy's, a possible nice interface for patch would be to have it have allow_pickle=None which would mean to not patch Packer, Unpacker, etc., so they'll all use their defaults for allow_pickle).

[arogozhnikov]

Hi @KennyChenBasis and @lebedov

My big +1 here.

Just found this package, very compactly written. My thought was to recommend this as a replacement for distributing data by pickles.
However I've hit the issue #50 (pickles are 'on' by default and there is no way to prevent that).

Regarding defaults: I for sure expect that loading has pickles off, and prefer that saving has pickling turned off (but that's an opinion, just saving does not introduce vulnerability per se).

[akapocsi]

This would be great to have in a pypi or conda-forge release!