Use yarn when installed

[Haroenv]

This should be checked here

relevant issue: facebook/create-react-app#896

[SamVerschueren]

It was suggested in np as well sindresorhus/np#98. It's a very awesome tool, but at that point we decided that it was a little bit too immature to rely on that. Not sure what the status on Yarn is today, they are working very hard though to push out new features and iron out bugs so yes maybe it could be added for xo-init.

It's also just installing one module here, so the use case is a bit different then in np's case where you want to be 100% sure that what you release really works.