add `overwrite` option

[TonyStarkBy]

In some cases we don't want copy file if it exists (to save file mtime)

[eddiemonge]

force is probably better here than overwrite but I'm fine with either

[TonyStarkBy]

Why "force"? By default it will work like current version - overwrite file every time when you run task.

[eddiemonge]

Actually, the check should probably be there anyway. Seems like a bug that it overwrites the file that is exactly the same