WIP: add headers support

[Carlotronics]

Fixes #4 and #121.

WIP: Still needs to find a way to compare given value to each header values (currently only the first one is taken)

[TwiN]

Personally, I would prefer if you just retrieved each header using response.Header.Get(), put it in a map, and call it a day.
Out of all the time I've used Go, this has worked for all of my use cases, and surely it'll be enough to cover most use cases that everybody else may have.

[Carlotronics]

The problem with response.Header.Get() is that you only get first header value. It will be enough in most cases, but not when working with response such as

HTTP/1.1 200 OK
[...]
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6)
User-Agent: AppleWebKit/537.36 (KHTML, like Gecko)

From RFC7230:

A recipient MAY combine multiple header fields with the same field
name into one "field-name: field-value" pair, without changing the
semantics of the message, by appending each subsequent field value to
the combined field value in order, separated by a comma.

Means that joining response.Header[headerKey] using a comma should not break any value (except for Set-Cookie which I should treat as a special case).

So the way I will implement it depends on the question: is it better to handle all cases and keep full response.Header[headerKey], or voluntarily exclude some of them for the sake of simplicity?

[TwiN]

Personally, I think we're fine with just checking the first header and keeping it at that.

If somebody opens up an issue in the future, that'll be a problem for then and there :P

[TwiN]

Since there has been no changes to this PR for over 6 months, I'm going to close it.
Thank you for the attempt, though, I appreciate it!