Option to set Umask

[miguelmarco]

The files that are created by the jupyterlab instance run under this spawner have permissions -rw-r--r--

For some use cases, it would make sense that the files of one user could not be readable by other users. The usual way to set a default umask (in bashrc or profile) does not work since thet apply at login time, whereas the spawner does not log in.

Could you add an option to set the umask for the jupyter process?