Adds noSave option for non-persistent auth

Hey,

I only need a temporary github token, which I use to get a travis token. I now added an option called `noSave` to allow usage of the module without saving.

Alternatively I guess it would also be possible to take the prompting and authing to another module, which [I also tried](https://github.com/finnp/ghauth/blob/master/index.js). But maybe this is a simpler solution?

Best,
Finn
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