lib/path.js
function rel(root, src, sep) {
  if(!root || !_.isString(root) || !src || !_.isString(src)) return
  let root_split = root.split(path.sep),
    src_split = src.split(path.sep)
  return _.join(_.difference(src_split, root_split), sep || '/')
}
function rel(root, src, sep) {
  if(!root || !_.isString(root) || !src || !_.isString(src)) return
  let root_split = root.split(path.sep),
    src_split = src.split(path.sep)
  return _.join(_.difference(src_split, root_split), sep || '/')
}
returns rel path of src from root