Fixes #429

This enables passing in a virtual filesystem compatible with the fs module. There are several matching the fs API:

• https://github.com/streamich/memfs
• https://github.com/jvilk/BrowserFS
• https://github.com/filerjs/filer (no sync API)
• https://github.com/isomorphic-git/lightning-fs (no sync API)

Example:

let glob = require('glob')
let memfs = require('memfs')

let volume = memfs.Volume.fromJSON({
  '/foo/bar/abc.txt': 'abc',
  '/foo/bar/def.log': 'def',
  '/foo/bar/xyz.txt': 'xyz',
})

glob.sync('/foo/bar/*.txt', { fs: volume }) // returns ['/foo/bar/abc.txt', '/foo/bar/xyz.txt']

Allows usage of glob in lots more places without requiring users to monkey-patch. With this change, glob could be used in the browser, with dropbox, S3, purely in memory like above (useful for testing) and many others.

After writing that issue up, I had a look at the code and the change required is very small so I thought I'd just open a PR.