Add support from reading from jimfs in-memory FS

[clintval]

Closes #59

Draft PR to use upstream's CI harness to make sure this works OK.

@cdimascio have any ideas for how I could test this?

[cdimascio]

Can you provide some background on the addition of jimfs. What is the use case and benefit of this change?

[clintval]

@cdimascio it's far from a critical feature for me. I wanted to use an in-memory file system for Groovy unit tests so I wouldn't have to write files in my temp directory. You can see that here:

https://github.com/fulcrumgenomics/nf-dotenv/blob/b4c64f84cbd237ee7be2729545e26b174f57bb24/plugins/nf-dotenv/src/test/nextflow/dotenv/MockHelpers.groovy#L36-L53

I'm using your library under-the-hood of a plugin I developed for a scientific workflow manager. Instead, I implemented my testing with real temporary files on a real filesystem.

[clintval]

This is the project that uses your library: nf-dotenv