Support for Gatsby v3

[samajammin]

Hey there! Thanks for this excellent plugin.

I noticed this warning after migrating our project to Gatsby v3:

warning " > [email protected]" has incorrect peer dependency "gatsby@^2.0.15".

Could we update this plugin to support v3? Looks like it might be as simple as updating the peer dependency:
https://www.gatsbyjs.com/docs/reference/release-notes/migrating-from-v2-to-v3/#for-plugin-maintainers

Cheers

[CollierCZ]

It's not a super important warning, but getting rid of it isn't hard. But I don't think this plugin is really maintained anymore. I made a fork to support symlinks and that one is updated for Gatsby 3.