Add option to choose a different package manager

[jariz]

We should give developers the freedom to choose their own package manager, rather than always use yarn if it's available:

https://github.com/jaredpalmer/razzle/blob/master/packages/create-razzle-app/lib/utils/get-install-cmd.js#L14

The only solution right now to this is globally removing yarn - which is not really a solution; I want to be able to keep supporting projects that use yarn, while favoring npm for new projects.
Preferably, we'd add a flag to create-razzle-app in which we can give a preference.

[stale]

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[stale]

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