Allow alternative fs to be provided via options

[dazinator]

The background on this is here: pahen/madge#186
I am working my way through the stack, and anything that directly uses fs I am seeing if we can allow an alternative fs implementation to be passed in. This allows some interesting scenarios like supporting virtual file systems, with things like unionfs and memfs.

Would you be open to a PR that allows an additional fileSystem option to be passed in, and if it is, will use that instead of the default fs ?

[dazinator]

Have submitted a PR as it seemed fairly straightforward.

[dazinator]

As no response I created a fork in the end and have put change there.