Add 'webpack.babel.exclude' config

[t-kelly]

What are you trying to accomplish with this PR?

Fixes #705

• Add 'webpack.babel.exclude' config which defaults to the same value as 'webpack.commonExcludes'. This value can be modified by slate.config.js so that it includes a pattern that does not omit certain node_modules, e.g. /node_modules\/(?!(MY-MODULE|ANOTHER-ONE)\/).*/.
• Refactor tests to mock fs.existsSync() instead of using mock-fs for mocking a .babelrc file.
• Add test for 'webpack.babel.exclude'
• Stylistic refactor of webpack/config/parts/babel.js
• Replace commonExcludes() method with direct use of the 'webpack.commonExcludes' config.

[lock]

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