A directory-category for terminal emulators

This is a mostly cosmetical commit, in the sense it doesn't change the contents
of any package, but reorganizes the overall Nixpkgs expressions.

Terminal emulators are an ubiquitous tool for any Unix user; even the beginners
are routinely familiarized to it. And, manifestly, there are many
implementations of terminal emulators out there, from those traditionally made
in C and C++ to those written in Haskell and Go.

Terminal emulators deserve more highlight. This commit does that by creating a
category for them.

pkgs/applications/terminal-emulators/rxvt-unicode-plugins/urxvt-perl/default.nix

@@ -0,0 +1,29 @@
+{ stdenv, fetchFromGitHub, wmctrl }:
+
+stdenv.mkDerivation {
+  name = "urxvt-perl-2015-01-16";
+
+  src = fetchFromGitHub {
+    owner = "effigies";
+    repo = "urxvt-perl";
+    rev = "c3beb9ff09a7139591416c61f8e9458c8a23bea5";
+    sha256 = "1w1p8ng7bwq5hnaprjl1zf073y5l3hdsj7sz7cll6isjswcm6r0s";
+  };
+
+  installPhase = ''
+    substituteInPlace fullscreen \
+      --replace "wmctrl" "${wmctrl}/bin/wmctrl"
+
+    mkdir -p $out/lib/urxvt/perl
+    cp fullscreen $out/lib/urxvt/perl
+    cp newterm $out/lib/urxvt/perl
+  '';
+
+  meta = with stdenv.lib; {
+    description = "Perl extensions for the rxvt-unicode terminal emulator";
+    homepage = "https://github.com/effigies/urxvt-perl";
+    license = licenses.gpl3;
+    maintainers = with maintainers; [ cstrahan ];
+    platforms = with platforms; unix;
+  };
+}