Allow to opt-out from installing dependencies

[bobisjan]

With ember-auto-import there is a possibility to avoid installing FastBoot dependencies at runtime, because they are bundled at compile time. This makes deploying on production less fragile, package manager independent, ...

@kratiahuja, @rwjblue, does it make sense to provide opt-out for installation step https://github.com/ember-fastboot/fastboot-s3-downloader/blob/master/index.js#L43?

Happy to provide PR,
Thanks

[kratiahuja]

Should this step just noop if apps are using ember-auto-import? Even if they are bundled at compile time, fastboot will still search in node_modules right ?