Add --skip-git option to create-snowpack-app

[rajasegar]

When using csa to create projects inside a mono-repo, creating an entirely new git repository is not necessary. It would be great to pass this as an option to csa like below:

create-snowpack-app my-react-app --template @snowpack/app-template-react --skip-git

[FredKSchott]

Oh wow, surprised by the quick +1s on this one! If this is something that people would like, I'm +1