Make repository importable as NUR (including an overlay)

https://github.com/nix-community/NUR is a Nix community project that
aims to make out of tree derivations more easily discoverable and
accessible to Nix users. Converting the nix-bitcoin repo to conform to
that style is a minor change and enhances reusability of its
components. For instance, I could slap on the clightning module more
easily onto my existing bitcoin node without having to redeploy the
whole as nixops driven installation. Having the repo in NUR style
would make that easier.

overlay.nix

@@ -0,0 +1,21 @@
+# You can use this file as a nixpkgs overlay.
+# It's useful in the case where you don't want to add the whole NUR namespace
+# to your configuration.
+
+self: super:
+
+let filterSet =
+      (f: g: s: builtins.listToAttrs
+        (map
+          (n: { name = n; value = builtins.getAttr n s; })
+          (builtins.filter
+            (n: f n && g (builtins.getAttr n s))
+            (builtins.attrNames s)
+          )
+        )
+      );
+in filterSet
+     (n: !(n=="lib"||n=="overlays"||n=="modules")) # filter out non-packages
+     (p: true) # all packages are ok
+     (import ./default.nix { pkgs = super; })
+