Update webpack.md to clarify what the config does

[wyuenho]

Remove cheerio as it's no longer optional depending on React version. I've also added an alternative config using the IgnorePlugin that will achieve the same effect.

[ljharb]

seems 👍

[nfcampos]

Looks great @wyuenho would you mind rebasing on latest master?

[ljharb]

@wyuenho any chance you'd be willing to reopen this and rebase it? I'd love to merge it.

[wyuenho]

Let me confirm with the people on #47 first. I see people are still using externals and I'm not sure why. It can probably be completely done away with.

[hpurmann]

I just had a problem with shallow when testing with Karma+Webpack (in mocha or when using mount, it's fine):

TypeError: (0 , _cheerio2.default) is not a function
    at ShallowTestWrapper.get (test/webpack-client-tests.js:69435:44)
    at ShallowTestWrapper.attr (test/webpack-client-tests.js:69351:19)
    at Assertion.<anonymous> (test/webpack-client-tests.js:56696:37)
    at Assertion.<anonymous> (test/webpack-client-tests.js:57115:20)
    at Assertion.ctx.(anonymous function) [as attr] (test/webpack-client-tests.js:11282:34)
    at Context.<anonymous> (test/webpack-client-tests.js:307560:32)

Using React 15.4.2 and enzyme 2.6.0.
The problem disappears when removing cheerio from externals.