Add neutral gender option for clients / Title

At present while not a required entry, if you want to add a title for a client, you only have the options Mr./Ms.. This is exclusive of non-binary identified people, and it would be good to add Mx (see https://en.oxforddictionaries.com/definition/mx), which in the database could be represented as an 'x'.

Another option would be to turn title (or salutation) into a free text field, but that might more complex in a multi-lingual environment.