ActiveFedora::Predicates.set_predicates allows you to set predicates without wiping out existing configs

Allows you to do things like this, usually in initializers:

```ruby
# Add :has_profile predicate for use in ActiveModel (RELS-EXT) relationships
ActiveFedora::Predicates.set_predicates("http://projecthydra.org/ns/relations#"=>{:has_profile => "hasProfile"})
```

or, more elaborate, setting some new predicates while overriding existing predicates

```ruby
ActiveFedora::Predicates.set_predicates({
                                          "http://projecthydra.org/ns/relations#"=>{
                                                  has_profile: "hasProfile"
                                              },
                                          "info:fedora/fedora-system:def/relations-external#"=>{
                                                  references: "references",
                                                  has_derivation: "cameFrom"
                                              },
                                           })
```