Print trailers

[mihaitodor]

Hey @andradei, I took a stab at fixing #100 (Support for Trailers).

This PR consists of 3 changes:

• the actual implementation
• small refactor to use strings.Builder instead of string concatenation for efficiency
• strings.Builder is not available before Go 1.10, so I bumped up the minimum required Go version too. If you're not OK with this, I can leave the refactor for a future PR.

For the sample HTTP server in the reported issue, the output now looks like this:

─Response headers──────────────────────
HTTP/1.1 200 OK
Content-Type: text/plain; charset=utf-8
Date: Wed, 03 Oct 2018 14:07:27 GMT
Atend1: value 1
Atend2: value 2
Atend3: value 3

[andradei]

Looks great!

I've seen the commit history isn't that active. Hopefully this can be reviewed and merged soon.