Add support for changing the default file mode using options pattern (fixes natefinch/atomic#7)

[sdassow]

Something about #12 didn't feel right or clean enough, so here's an alternative approach using the options pattern instead.
While this increases complexity a bit, it comes with the benefit of a clean interface in case of further options.

[sdassow]

Now after renaming the option from FileMode to DefaultFileMode, it would be possible to add a FileMode option with the same semantics a FileMode in the function signature would have (like os.WriteFile() for example), which in turn could be an answer to #7 as well.