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

@@ -2,10 +2,14 @@
 # executing commands, will be sourced when starting as an interactive shell.
 
 # Load plugin scripts
-plugin-load() { source ~/.config/zsh/$1/$1.plugin.zsh }
-plugin-load zsh-autosuggestions
-plugin-load zsh-history-substring-search
-plugin-load zsh-syntax-highlighting
+source-plugin() {
+  [ -d ~/.config/zsh/$1 ] && \
+    source ~/.config/zsh/$1/$1.plugin.zsh || \
+    echo "zsh plugin not found: $1"
+}
+source-plugin zsh-autosuggestions
+source-plugin zsh-history-substring-search
+source-plugin zsh-syntax-highlighting
 
 # Disable non end-of-line autosuggest accept widgets
 ZSH_AUTOSUGGEST_ACCEPT_WIDGETS=(end-of-line vi-end-of-line)