Post create hook

Hi,

this is my first PR for this project. We are currently setting up an analysis cluster, using motley cue for the account creation and ldap as a local backend. To create additional directories for a new user (namely setting up a new zfs dataset including quota), I added the functionality to run an arbitrary script after creating a new ldap user. Currently, I have implemented this new feature only for the ldap backend.

Do you think such a feature should be added?

Best Wishes Sebastian