Switch verbosity macros to use group argument

[quinnj]

As noted in this comment, there's currently a semantic error with the verbosity macros where even if my verbosity is explicitly set to 0 or 1, but my log level is Info, then a message like:

@errorv 2 "more detailed error message than you normally want to see"

will still be logged, since the current logic just does Error - 2.

With the change proposed in this PR, we get the following change in behavior:

• If LoggingExtras.withlevel or an alternative filter doesn't inspect the group.verbosity argument, then the verbose macros act just like the normal logging macros
• If withlevel is used, then the above case acts as expected; i.e. the @errorv 2 msg case only logs if withlevel(Info; verbosity=2) is set.

I opted to wrap the verbosity level in a new LoggingExtras.Verbosity struct so that if the group argument is ever used for something else as an Int, it won't accidentally conflict with the verbosity filtering. We'll still clobber that argument if anyone else tries to use it, but it shouldn't affect cases where people aren't using the verbosity macros.

Fixes JuliaWeb/HTTP.jl#938.

[oxinabox]

I am convinced.
I am not sure why i was so opinionated the other way before.

I mean i do still see merit to my argument, but not as much.

(Aside: I continue to thinkg that the group should be something that acts kind of like an AbstractSet, so you can add multiple things and then check if any things are contained in it. But that is orthogonal to this PR)

I only have one small comment, (applies to multiple bits of the code)
address that as you see fit and merge when happy.

I think lets not tag a release with this, since as you say it is breaking.
but with this done we will be able to tag 1.0
(once we also do #70)