added --use-npm flag

[schoening]

I have added a --use-npm flag that skips the yarn install attempt when running npx create-keystone-app

I tried to get rid of duplication and refactored it into their own functions. Please feel free to give me feedback if this structure is not to your liking!

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[dcousens]

Thanks @schoening , but I think will reject this in favour of what is being discussed in #214 (removing the package manager specific parts)