Request: custom request headers as argument

[vzamanillo]

Some websites use WAF for protection, in that cases it is possible that we need some additional headers (Host, Cookie.. etc) to avoid 403 errors.

As a proposal, a new command line argument should be enough, with JSON format or whatever to easily transform it to a dict and pass it to the request.

[OwenChia]

good idea, I'll look at it (maybe take times, quite busy these days). also, feel free to send pr.