add --update option. minor fixes

lib/utils/functions.sh

@@ -3,4 +3,8 @@
 # this is bad, but `declare -f -F` is not portable
 is_function () {
   type $1 2> /dev/null | grep -q 'function$'
-}
+}
+
+command_exists () {
+  type $1 > /dev/null 2>&1
+}

lib/utils/helpers.sh

@@ -25,4 +25,4 @@ unique_id () {
 
 print_prefixed () {
   printf "$2" | sed -e "s/^/$1/g"
-}
+}