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/default.nix

@@ -0,0 +1,22 @@
+{ callPackage }:
+
+{
+  autocomplete-all-the-things = callPackage ./urxvt-autocomplete-all-the-things { };
+
+  bidi = callPackage ./urxvt-bidi { };
+
+  font-size = callPackage ./urxvt-font-size { };
+
+  perl = callPackage ./urxvt-perl { };
+
+  perls = callPackage ./urxvt-perls { };
+
+  resize-font = callPackage ./urxvt-resize-font { };
+
+  tabbedex = callPackage ./urxvt-tabbedex { };
+
+  theme-switch = callPackage ./urxvt-theme-switch { };
+
+  vtwheel = callPackage ./urxvt-vtwheel { };
+
+}