Add Greg Maxwell's banlist to bitcoind postStart

pkgs/banlist.nix

@@ -0,0 +1,15 @@
+{pkgs}:
+
+with pkgs;
+stdenv.mkDerivation {
+  name = "banlist";
+  src = ./banlist.sh;
+
+  unpackPhase = "true";
+  installPhase = ''
+    mkdir -p $out
+    mkdir -p $out/bin
+    chmod +wx $src
+    cp $src $out/bin/banlist
+    '';
+}