Add support for custom Header

[melbahja]

I will test this later today, thank you :smile:

[melbahja]

I did some changes:

• apply headers to all requests
• trim header value in case of got --header "key: value" --header "key2: value2"

you can check the diff here: https://github.com/melbahja/got/compare/v0.4.0...poldi1405-master

Thanks :)