Windows - Long Path Support (260 chars)

Currently, if input/output paths for **opusenc** are too long or Windows, **opusenc** is not able to find the file reporting error:

> No such file or directory

This seems to be due to lack of support for long paths. 

Would it be possible to fix this for Windows builds?

Similiar issues:
https://trac.ffmpeg.org/ticket/8885
https://github.com/staxrip/staxrip/issues/226