Upgrade gli to 2.7.0
gli2: use builtin version and help
gli2: set switches non-negatable

Conflicts:
	lib/brightbox-cli/gli_global_hooks.rb
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.