install: check dependencies before running

install.sh

@@ -17,6 +17,7 @@ main() {
         *) usage; exit 1;;
     esac done
 
+    check_dependencies
     greeting
     make_dirs
     stow_home
@@ -25,6 +26,15 @@ main() {
     nvim_plugins
 }
 
+check_dependencies() {
+    for cmd in stow readlink; do
+        if ! command -v "$cmd" >/dev/null 2>&1; then
+            error "Dependency \`$cmd\` not found."
+            exit 1
+        fi
+    done
+}
+
 greeting() {
     dry_run && {
         warn "Performing dry run (use -f to actually make changes)."