Package nextflow plugins with nf-core download

[fa2k]

Description of feature

nf-core pipelines download is great for running pipelines in places without internet access. The only thing missing is usually the nf-validation or nf-schema plugin.

This feature request is that Nextflow plugins could be bundled with the downloaded pipeline directory. Then the only thing needed to run the pipeline would be a working nextflow and singularity.

Now it's possible to get pipelines to work offline by also transferring $HOME/.nextflow from a computer with the plugin installed, but this is not very convenient.