Add support for default request items in config file

I searched through the open (and closed) Issues looking for a way to set a default "User-Agent" and other request items (Headers, URL parameters etc) and didn't find anything really useful. Closest thing I found was using the --session option [1], which I didn't try myself, but didn't look like a clean way to achieve what I wanted. Thus, I added a new option in config.json file, to support setting of default request items.

All tests completed successfully, except for "test_timeout_exit_status" for which there is an open bug report [2].

[1]: https://github.com/jkbr/httpie/issues/180
[2]: https://github.com/jkbr/httpie/issues/185