Add after_install_package hook

bin/ruby-build

@@ -13,6 +13,7 @@ install_package() {
   extract_package "$package_name"
   cd "$package_name"
   build_package "$package_name" $*
+  after_install_package "$package_name"
   cd "$cwd"
 }
 
@@ -69,6 +70,10 @@ build_package_copy() {
   cp -R . $PREFIX_PATH
 }
 
+after_install_package() {
+  local stub=1
+}
+
 usage() {
   echo "usage: ruby-build DEFINITION-PATH INSTALLATION-PREFIX"
   exit 1