Provide --overwrite argument to overwrite code in place

[johnthagen]

As part of a build process, it can be helpful to overwrite the .py file in place. On *Nix, you can use > shell redirection, but this isn't portable to all platforms, such as Windows.

$ pyminifier --overwrite hello.py

[Daniel Flook]

This has been implemented in 2.6.0, where it is spelled --in-place.