Make puppeteer an optional dependency

[andrewiggins]

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Continuing our discussion from #54 and slack, wanted to see what people think about this.

Pros:

1. Reduces installation size and time of karmatic and its deps
2. Still allows consumers to use puppeteer if they want

Cons:

1. Assumes users already have Chrome installed
2. Breaking change

Probably other thoughts I haven't considered.

I'm not personally attached to any approach (normal dep, peer dep, secret optional dep), but just wanted to get consensus before our next release. What do y'all think? @developit, @marvinhagemeister

[developit]

I think I still prefer peerDep, though perhaps I need to rethink that now that npm is installing those by default...