feat: expose libcurl timings in the faraday env

[Roguelazer]

This collects all of the detailed timings from libcurl (via Typhoeus) and exposes them as response.env.custom_members[:typhoeus_timings]. We're using this internally, and I thought other folks might find it helpful, too.

Example:

[1] pry(main)> resp = Faraday.new { |b| b.adapter :typhoeus }.get "https://www.example.com"
ETHON: Libcurl initialized
ETHON: performed EASY effective_url=https://www.example.com/ response_code=200 return_code=ok total_time=0.082656
[2] pry(main)> puts resp.env.custom_members[:typhoeus_timings]
{"appconnect"=>0.063223, "connect"=>0.023084, "namelookup"=>0.005523, "pretransfer"=>0.06355, "redirect"=>0.0, "starttransfer"=>0.081755, "total"=>0.082656}

[dleavitt]

Thanks, seems legit! Before merging, will probably want to:

1. Placate Rubocop (which I've just made easier)
2. Add a smoke test for it, probably about here
3. Maybe do the :typhoeus_timings keys as symbols instead of strings?

[Roguelazer]

@dleavitt eminently-reasonable suggestions; I believe I've done them all, although the smoke test is not very good, since this uses webmock and webmock doesn't expose these fields. I filed bblimke/webmock#1038 to address that, but it looks like there hasn't really been any activity on webmock for a few months...

[dleavitt]

Hooray, thanks for the effort! I'll take a look soon.