Support Nuxt.js >= 2.0.0

[rclement]

This PR aims at fixing the compatibility with Nuxt.js >= 2.0.0:

• New dependencies: extract-text-webpack-plugin, friendly-errors-webpack-plugin
• New dev-dependency: @nuxt/common
• Replace usage of r() and urlJoin() from nuxt using the @nuxt/common package
• Update webpack config
  • Replace module.loaders with module.rules
  • Resolve CHUNK_FILENAME as a string, depending on Nuxt build configuration

This fixes issues #104 and #105.
This can be tested using the @rmnclmnt/nuxt-netlify-cms package.

[medfreeman]

thanks! also closes #60

[rclement]

@medfreeman here are the latest enhancements to this PR:

• I merged back your upstream changes in master branch to resolve conflicts in package.json
• Following the advice in Compatibility with nuxt2 (nuxt-edge) #60, I replaced extract-text-webpack-plugin (deprecated) with mini-css-extract-plugin for webpack v4 compatibility

Feel free to comment on this!
Thanks!

[medfreeman]

Hi !
Sorry but it seems my circleci settings do not build forked pull requests.
Thanks for the work, but the commits aren't enough since they do not pass tests at all.
I pretty much finished the work while keeping your commit in the rclement-support-nuxt-2.0 branch.

I used extract-css-chunks-webpack-plugin, as nuxt 2.0 does.

[medfreeman]

I'm testing the example atm, and will update test snapshots then merge to master.
I'll also keep the previous code in @nuxt1 branch.

[medfreeman]

Oh, it seems i need webpack bar, and a way to register the admin compiler in nuxt compilers to streamline the output also.

[medfreeman]

closing in favor of #111