add add_extension script

scripts/add_extension.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+EXT_NAME=$1
+EXT_VERSION=$2
+
+cd "${WIKI_SOURCE}/extensions"
+
+if [[ -e "${EXT_NAME}" ]]; then
+    cp -Rv "${EXT_NAME}" .
+    cd "$(basename ${EXT_NAME})"
+else
+    git clone "https://gerrit.wikimedia.org/r/mediawiki/extensions/${EXT_NAME}"
+    cd "${EXT_NAME}"
+fi
+
+if [[ "${EXT_VERSION}" ]]; then
+    git checkout -b "${EXT_VERSION}" "origin/${EXT_VERSION}"
+fi
+
+if [[ -e "composer.json" ]]; then
+    composer install --no-dev
+fi