.functions: Add gi

This is an easy way to install Grunt plugins and add them as `devDependencies` to `package.json`.
Example:

    gi contrib-watch contrib-uglify zopfli

Closes #191.

.functions

@@ -239,3 +239,10 @@ function unquarantine() {
 		xattr -r -d "$attribute" "$@"
 	done
 }
+
+# Install Grunt plugins and add them as `devDependencies` to `package.json`
+# Usage: `gi contrib-watch contrib-uglify zopfli`
+function gi() {
+	local IFS=,
+	eval npm install --save-dev grunt-{"$*"}
+}