Specify downloads folder path to allow Jenkins to cache CEF and Node downloads

* Remove the default `grunt setup build` from `npm install` (TODO update wiki). This allows us to control when we call `grunt setup` to pass new args `--downloads=../downloads`.
* New `--downloads` option allows Jenkins jobs to specify a path to store CEF and Node downloads so that when the workspace is wiped clean, we can access cached downloads quickly instead of downloading the same binaries for each build trigger
* TODO update Jenkins jobs to add `grunt setup --downloads=../downloads`. Already tested by @jasonsanjose.