Handle foo=True args in saner way

[ekimekim]

Right now the behaviour is for foo=True to create a --foo arg with store_false, ie. so that --foo means foo=False. This is very counterintuitive.

neithere#73 has some thoughts on this, and I've implemented a similar thing in my own usage previously as an explicit custom Action that adds --foo and --no-foo args.

Open questions:

• Do we want both --foo and --no-foo, or do we just want --no-foo if the default is foo=True
• There should still be a way to opt into both --foo and --no-foo, especially if the default is dynamic based on other conditions - the one that comes up a lot personally is something like --color --no-color with default being sys.stdout.isatty().
• How do we reconcile this with existing behaviour without breaking backwards compatability too badly?
• Does it make sense to combine this with short args? eg. -f=true/false or -f=yes/no (probably not - if the user wants this they can explicitly use choices=["yes","no"])