Contents cache demo

Issue: 341

chrome extention

when you build the project arose the following problems:

there is no possibility to change the name of the directory _nuxt (_nuxt -> сustom_dir / in extensions are not valid user names of files and directories with an underscore)
using the mode: 'hash', incorrect references are generated, they still generated for mode: 'history'
how to use CopyWebpackPlugin only stage generate?

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