Add -g option.
Merge pull request #139 from mashiro/add-global-option

Add -g option.
stdlib warning fixes using shellcheck

see https://github.com/koalaman/shellcheck
Allows to specify which version of python to use in layout_python
Makes `layout python3` a shortcut for `layout python python3`
Use a different virtualenv per python versions

This changes the path of the virtualenv folder. The old path is detected and
re-used to avoid unecessary re-installation.