Allow disabling updates across major versions with Github strategy

[padraic]

Note: This PR should be closed without merging if #43 is approved for a 2.0 release.

A little cross-pollination from #37 to keep features as aligned as possible within the existing behaviour.

[keradus]

Hi ! looks like a change i could benefit from ;)
What is the state here?

[theofidry]

The state is I'm a bit short of time for Christmas and NY so I'm trying to finish to write an article with a roadmap regarding the PHAR-related projects (which include this one) and then I'll tackle them.

Not sure where this one will be in the priority list, I think I'll want to finish PHP-Scoper integration with box first.

Also I find the current usage of this library a bit cumbersome for simple cases so I would like to change a couple of things there as well (see #53).

TL:DR; either wait for my post (in 1-2 weeks) or a couple of month, I can make sure to ping you for this when it's done :)

[keradus]

Don't worry, I'm not rushing you ;)
Just asking about current state of art here, as this PR was approved half year ago ;) (also, it is not issue of christmas time for that ;P)

thanks for feedback !