Add ability to set mtime field to exact value (a number or UTC time or Unix timestamp)

Currently `mtime` takes in a `Date`, but JS `Date` objects are notoriously quirky. One major problem is the fact that JS `Date` has time zone information, but archive file formats typically do not. It would be nice if we could pass to `mtime` an actual exact value (as a JS integer/number) or an exact to be placed in the archive.