Add header and basic auth support on fetch URLs

[int128]

% go run main.go --debug --request-header=x-foo:bar --request-header=x-foo:baz --request-auth=alice:bob https://www.example.com/image.jpg
2018/09/17 03:15:05 The following 1 items will be uploaded:
  1: https://www.example.com/image.jpg
2018/09/17 03:15:05 Using token in ~/.gpupconfig
2018/09/17 03:15:05 Queued 1 item(s)
2018/09/17 03:15:05 [REQUEST] GET https://www.example.com/image.jpg
GET /image.jpg HTTP/1.1
Host: www.example.com
User-Agent: Go-http-client/1.1
Authorization: Basic YWxpY2U6Ym9i
X-Foo: bar
X-Foo: baz
Accept-Encoding: gzip

2018/09/17 03:15:06 [RESPONSE] GET https://www.example.com/image.jpg
HTTP/2.0 404 Not Found