#181 capability to open/save gz files

[strassenbahn]

As there are many text files that come in gzipp'ed format I was looking for a way to open (and even save) them directly in nedit.
I used the zlib and recognize the zipped files by the the file ending '.gz'. So does also work for new files to create using nedit. detecting gz files by file contents has some further implications which requires more effort.