Support outputting the metadata of a response

[ducaale]

This PR adds support for outputting response's elapsed time as part of a new metadata section which can be enabled via either --print=m, a double invocation of the verbose flag i.e -vv, or via the --meta flag (similar to --headers and --body).

The duration of a response's elapsed time begins from when a request is made and ends when the response has been fully printed.

$ xh -vv httpbin.org/status/418

GET /status/418 HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate, br
Connection: keep-alive
Host: httpbin.org
User-Agent: xh/0.15.0

HTTP/1.1 418 I'm a teapot
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Content-Length: 135
Date: Sat, 12 Mar 2022 21:30:13 GMT
Server: gunicorn/19.9.0
X-More-Info: http://tools.ietf.org/html/rfc2324


    -=[ teapot ]=-

       _...._
     .'  _ _ `.
    | ."` ^ `". _,
    \_;`"---"`|//
      |       ;/
      \_     _/
        `"""`


Elapsed time: 0.37325s

Ticks another item from #4

[ducaale]

I think this PR is ready now. I will merge it and we can fix any found issues later