stack.yaml: switch from Stackage LTS to Nightly

Let's use a moderately recent compiler like ghc-8.10.3
instead of the old 8.8.x branch for development. Same goes
for our dependencies; development should take place with
recent versions of our dependencies.

stack.yaml

@@ -1,7 +1,24 @@
-resolver: lts-16.22
+resolver: nightly-2021-01-05
+
+flags:
+  xmonad:
+    generatemanpage: true
 
 packages:
 - ./
 
 extra-deps:
 - X11-1.9.2
+- pandoc-2.11.3.2
+- citeproc-0.3.0.3
+- commonmark-0.1.1.2
+- commonmark-extensions-0.2.0.4
+- commonmark-pandoc-0.2.0.1
+
+nix:
+  packages:
+    - zlib
+    - xorg.libX11
+    - xorg.libXrandr
+    - xorg.libXScrnSaver
+    - xorg.libXext