Improve & rename plugin sourcing function

Rename `plugin-load` to `source-plugin` and add a check for the plugin
directory, a message is displayed if it does not exist.

zshrc

@@ -3,9 +3,11 @@
 
 # Load plugin scripts
 source-plugin() {
-  [ -d ~/.config/zsh/$1 ] && \
-    source ~/.config/zsh/$1/$1.plugin.zsh || \
+  if [ -d ~/.config/zsh/$1 ]; then
+    source ~/.config/zsh/$1/$1.plugin.zsh
+  else
     echo "zsh plugin not found: $1"
+  fi
 }
 source-plugin zsh-autosuggestions
 source-plugin zsh-history-substring-search