Support to extract 7z

[souenzzo]

Just 7z x file.7z. And, why not alias 'extract=uncompress'?

$ betty extract file.7z 
Betty: I don't understand. Hopefully someone will make a pull request so that one day I will understand.
$ betty uncompress file.7z 
Betty: Running mkdir file &&  tar -zxvf file.7z -C file

gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now