Add `extends` option

[SamVerschueren]

Add possibility to extend the configuration with the extends option like ESLint.

[SamVerschueren]

Since #13 is implemented, this one should be quite easy to do.

[voxpelli]

Much 👍 on this

Just found this project after @sindresorhus starred it and would like to look into using it in all of our internal projects, but since they are so many, I would prefer to have a single shared internal config, like I currently do with eslint-config-standard – wrap it in an internal module that adds some extras and then have all internal projects rely on that.

Is this still something you are interested in implementing @SamVerschueren?

[SamVerschueren]

Yes, would be happy to have this feature. Just not sure if I can find the time to do it so help is always welcome.