Provide a not install option for using yarn

[leiserfg]

I love to use yarn (better for offline working, faster installations...), but you are making me install with npm.

[yoshuawuyts]

Heya, thanks for opening this issue! These days npm is on par, if not faster than yarn. For the installation process we needed to pick something, and npm is the safe choice because everyone has it installed. Right now we don't have any options available, we probably want to keep it that way for a while. Hope that makes sense!

[bcomnes]

We could maybe add a --yarn flag