Move the vary util to a separate repo

[jonathanong]

I'm sure it's going to come up again. Plus, if like to use it :)

[dougwilson]

I'm already doing it :)

[jonathanong]

You're a magician

[dougwilson]

I wrote it in method-override, then immediately said "this needs it's own repo" as soon as I had tests that specifically tested edge cases in vary... because the module's should care that it works right... it should Just Work.

[Fishrock123]

👍

[dougwilson]

Vote for module name. So far I like vary-on or vary-by

[dougwilson]

...or vary-it lol. This module will be used on server and client-side, so it does header setting and header getting.