Fix empty object when adding runtimeCaching custom plugins with workbox-webpack-plugin

[tsirlucas]

R: @jeffposnick @philipwalton

Fixes #1597

As discussed on the issue, I've used this library by Yeoman instead of JSON.stringify because it doesn't handle functions, resulting in an empty object when adding custom plugins with workbox-webpack-plugin. Also, I've made some changes and now I'm using a regex based on this one to remove comments once the object is parsed as a string.

Also added some test cases.

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[coveralls]

Coverage decreased (-0.001%) to 86.531% when pulling e3c49ad on tsirlucas:master into 096e448 on GoogleChrome:master.

[tsirlucas]

Hey @jeffposnick just got back to this and got some cases that the strip comments regex wouldn't work correctly like when parsing a function with url strings into it. I'm working on it and will make a push to the branch ASAP.

[tsirlucas]

@jeffposnick I ended up adding the strip-comments dependency because there are too many cases to handle when removing comments from JS code like this. The complexity impact would be kinda big. Also added the test case.

I'm open to suggestions if you don't think that adding the dependency is the best approach.

[jeffposnick]

Awesome—this approach looks good to me, and thanks for adding test cases!

Since this is build-time tooling, I'm not concerned about introducing the extra dependencies if they lead to correct code.