Feature Request: ability to add an arbitrary filter to the user search

[cowboyd]

As an escape hatch, it would be nice to narrow the search at the LDAP level with very particular filters. We'd like to propose adding a userSearchFilter that would be "anded" together with all the other filters to make this possible. We are also totally willing to submit a PR implementing this functionality.

[shaozi]

This sounds like a good idea. Only thing I will think of is if this filter somehow is not checked it may be a source of injection. If you can provide a PR that will be great!