Add a posibliity for a pre-start hook

- The start.sh script does different operations on users, and
  sometimes one may need to do operations *after* that is done (as
  opposed to /usr/local/bin/start-notebook.d/ which is run before).
  This provides that possibility.  (One use case is hooks which must
  be run after the users are set up.)
- To be technically perfect, one might want to use something similar
  to /usr/local/bin/pre-start-notebook.d/.  But let's start with the
  simplest thing possible for now - the expected use of this command
  is only if people use NB_USER, etc in a way that requires small
  follow up commands to be run.
- If notebook is running as root, this allows users to run arbitrary
  commands pre-sudo.  Consider security model.

base-notebook/start.sh

@@ -89,6 +89,7 @@ if [ $(id -u) == 0 ] ; then
 
     # Exec the command as NB_USER with the PATH and the rest of
     # the environment preserved
+    eval "$NB_PRE_START_HOOK"
     echo "Executing the command: $cmd"
     exec sudo -E -H -u $NB_USER PATH=$PATH XDG_CACHE_HOME=/home/$NB_USER/.cache PYTHONPATH=$PYTHONPATH $cmd
 else
@@ -133,5 +134,6 @@ else
 
     # Execute the command
     echo "Executing the command: $cmd"
+    eval "$NB_PRE_START_HOOK"
     exec $cmd
 fi