Bump mini-css-extract-plugin from 2.5.3 to 2.6.0 (#358)

Bump mini-css-extract-plugin from 2.5.3 to 2.6.0 (#358)

Bumps [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) from 2.5.3 to 2.6.0.
- [Release notes](https://github.com/webpack-contrib/mini-css-extract-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/mini-css-extract-plugin/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/mini-css-extract-plugin/compare/v2.5.3...v2.6.0)

---
updated-dependencies:
- dependency-name: mini-css-extract-plugin
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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