Allow convert() to take multiple paths?

[runningskull]

Right now there's no good way to combine multiple fields into one. A nice way to allow that might be for convert() to take multiple paths, which would cause it to get multiple paths and pass them all into the conversion function.

Usage would look like

reformat.convert('name.first', 'name.last', (first, last) => {
  return first + " " + last
})