Upgrade to gli 2.5.6

Needed a couple of tweaks, but nothing major
gli2: use builtin version and help. set switches non-negatable
Use negatable switch for base64 encoding

This changes the behaviour of the options a bit which isn't 
backwards compatible, but it's a less used option so I think it's ok.

 --no_base64 becomes --no-base64 which is easy enough

but -e now means *do* encode base64 (and is default), which 
inverts the old behaviour. Bad form to change this but again, a rarely 
used option and the new behaviour is more consistent.
Workaround for a GLI bug where on_error is called for --help

We're basically re-implementing it here to avoid outputting an ugly
error message. Can probably be removed when GLI bug #150 is fixed.

https://github.com/davetron5000/gli/issues/150