style: Add pre-commit checks and code cleanup

This change adds an optional pre-commit configuration that can be used
to keep the code style clean.  I've also run it across all files and
fixed numerous whitespaces issues.

To use it, if wanted, just clone / pull the repo as normal, go into the
folder and run:

```
pre-commit install
```

From that point on, when running `git commit`, it will run the checks
on any changed files.

Feel free to ignore this PR if you're not interested.

notify_gotify.sh

@@ -8,18 +8,18 @@ send_notification() {
     Updates=("$@")
     UpdToString=$( printf "%s\n" "${Updates[@]}" )
     FromHost=$(hostname)
-    
+
     # platform specific notification code would go here
     printf "\nSending Gotify notification\n"
-    
+
     # Setting the MessageTitle and MessageBody variable here.
     MessageTitle="${FromHost} - updates available."
     MessageBody="Containers on ${FromHost} with updates available: ${UpdToString}"
-    
+
     # Modify to fit your setup:
     GotifyToken="Your Gotify token here"
-    GotifyUrl="https://api.gotify/message?token=${GotifyToken}"   
-    
+    GotifyUrl="https://api.gotify/message?token=${GotifyToken}"
+
     curl \
         -F "title=${MessageTitle}" \
         -F "message=${MessageBody}" \