[Closes #39] Added request/6.

Added a generic `request`, in the spirit of [gun](https://github.com/extend/gun) where there is no [`get`](https://github.com/extend/gun/blob/master/src/gun.erl#L159) function that lets the user specify a body, but there's a [`request`](https://github.com/extend/gun/blob/master/src/gun.erl#L207) function that provides absolute freedom.