allow setting up support for arbitrary pastebins

Currently support for additional pastebins is done through a Launchpad bug (wishlist request).

In my workplace, we have a local, private pastebin that we host ourselves. This is not published to the outside world, and as such it seems to me like it doesn't really have its place in the pastebinit source per se.

Instead, I think there should be some kind of configuration repository, for example '/etc/pastebinit/bins.d/' which could contain the parameters needed to support additional pastebins. This could also likely avoid local patches made to the source code on a system to be overwritten in the case of a package update.