Rework request customisation

I’d independently discovered Variadic configuration, but variadic
functional configuration seems to be better.

See http://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis
and http://commandcenter.blogspot.com.au/2014/01/self-referential-functions-and-design.html
Merge pull request #15 from alphagov/feature/request-functional-options

Rework request customisation