Cookie support (sessions) added

[tomikoski]

I've added cookie support, can be used something like:

echo target.tld | mantra -ua 'go-client/1.0' -c 'SESSION=1; admin=false; something=else'

Tested using macOS:

# terminal1:
GOOS=darwin GOARCH=arm64 go build -o build/mantra-arm64-darwin
echo "http://localhost:9999" | ./build/mantra-arm64-darwin -c 'testcookie=1' -ua 'test-client'

# terminal2, outputs:
$ nc -l 9999
...
...                                                                                                                        ─╯
GET / HTTP/1.1
Host: localhost:9999
User-Agent: test-client
Cookie: testcookie=1
Accept-Encoding: gzip
...
...