Add access logging functionality

[fredrikekre]

This adds access logging functionality to HTTP.listen/HTTP.serve. This includes a string macro that parses NGINX style format strings (chose that since it is pretty readable, self-explanatory, and looks more like Julia code compared to e.g. Apache format strings).

Example usage:

HTTP.listen(handler, host, port; access_log = logfmt"$remote_addr $request")

which would then log

[ Info: ::1 GET / HTTP/1.1
[ Info: ::1 POST /hello/post HTTP/1.1
[ Info: ::ffff:7f00:1 POST /hello/post HTTP/1.1
[ Info: ::ffff:7f00:1 HEAD /hello/post HTTP/1.1

etc.

Originally I thought this had to be implemented "on the inside" in order to get access to e.g. number of written bytes etc, but (as done in this PR) those things can technically be stored in the http::Stream object and you could do:

HTTP.listen(host, port) do http
    # handle stuff
    do_log(http) # do logging like this instead
end

but it is pretty convenient to just have to pass the access_log keyword and it does not really cost anything. In addition, if you want manual control you can still do it like above.

[codecov-commenter]

Codecov Report

Merging #713 (ed8afb1) into master (d4a7bbe) will increase coverage by 0.48%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #713      +/-   ##
==========================================
+ Coverage   76.04%   76.52%   +0.48%     
==========================================
  Files          36       37       +1     
  Lines        2371     2437      +66     
==========================================
+ Hits         1803     1865      +62     
- Misses        568      572       +4     

Impacted Files	Coverage Δ
src/HTTP.jl	97.82% <ø> (ø)
src/access_log.jl	93.22% <93.22%> (ø)
src/Servers.jl	81.25% <100.00%> (+0.35%)	⬆️
src/Streams.jl	96.20% <100.00%> (+0.09%)	⬆️

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[oxinabox]

Basically LGTM.
Other than a few things, most importantly making sure it doesn't break existing code if it doesn't have to.

[fredrikekre]

Thanks for the review, updated all of it I think (and fixed tests on Windows).

[oxinabox]

You have merge rights here, yes?
Merge when you are happy

[quinnj]

Yeah, sorry for the slow review; this looks great!