Avoid express overwriting you package.json file if it already exists

If you start a new project by running `npm init` and then thinks you want a express boilerplate and writes `express` and init in the current directory, your new package.json will be overwritten by express, because it does not look for the file first.

I thought about making a function similar to write(), so it would be exists(), but the exists command is already so short, and then you would have to pass the write command as a callback, that would make it more messy.

Was i right?