Allow passing a file to the plugin, as the destination for injected props

[argyleink]

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see https://github.com/argyleink/open-props/issues/315

[soluml]

Would love to see this! Would be incredibly useful with CSS Modules.

Seems like if this option was passed you could defer writing to each file and once all files were done you could write to the provided file path if it exists (or error).