Eilara/extra args for get method

LWP related Perl methods (e.g. WWW::Mechanize::get) always let you pass
through arguments, to be picked by the various layers along the call
flow, in a chain-of-responsibility style.

This allows you to pass custom headers, for example, no matter how many
abstractions between the caller and the final receiver. In this case,
somewhere beyond HTTP::Request.

We should do the same.