Upgrade dependencies and parametrize snapshots directory name

[rigelk]

resolves #76 #77 #73
closes #79 #78

note: I didn't touch the service worker because I think it ought to be written by hand, using the one produced as a template. But maybe I should parametrize it too?

[xuset]

The changes look good so far! Thanks for the PR!

I remember a while ago if dev dependencies were included in the package-lock.json then npm install <module> would also install the dev dependencies. Do you know if this is still the case?

[rigelk]

@xuset sorry about the comment - I replied to the wrong project PR on mobile and that didn't go as expected :/

[rigelk]

@xuset to answer your question, npm install installs both devDependencies and regular dependencies. But when one installs planktos - or any module - as part of their dependencies, only dependencies are taken into account, regardless of the package-lock's inclusion of devDependencies. At least that's what I get from here.