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

[bershanskiy]

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.