Various changes and improvements (#1)

* Use mod.conf for (soft) dependencies and description.
* Deletes deprecated `depends.txt` and `description.txt`.
* Add GitHub workflow to LuaCheck the code
* Fix minor LuaCheck warnings

.github/workflows/build.yml

@@ -0,0 +1,16 @@
+name: build
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@master
+    - name: apt
+      run: sudo apt-get install -y luarocks
+    - name: luacheck install
+      run: luarocks install --local luacheck
+    - name: luacheck run
+      run: $HOME/.luarocks/bin/luacheck ./