Promise support for `router.param`

[jonchurch]

This is a reminder to myself (or anyone else) to implement this and give a smooth upgrade path to users of express-async-errors

davidbanham/express-async-errors#28

[jonchurch]

@dougwilson Can you assign this issue to me? I don't seem to have perms to do that in this repo.

[wesleytodd]

@jonchurch are you still interested in working on this? If you are, I would love to figure out how we can land holistic promise support before router@2 happens. I have this one in flight from a while back as well: #75

[jonchurch]

Yeah Im interested. Need to dig up old stuff from the past but I had reached out to the maintainer of a popular package that monkey patched router with promise support.

If I can coordinate a release there to not crash when people use it with a native promise supporting router that would be great

[jonchurch]

@wesleytodd oh this merged but the issue wasn't closed #92