The name `CustomInterface` is too similar with `CustomPageInterface` and
the upcoming `CustomSnippetInterface` and their custom interfaces
settings file. Better rename it to `AdditionalInterface` so there is
less confusion.

The settings file will contain an override for the default snippet
interface, in addition to the default page interface, so we're renaming
it up front here.

Fixes torchbox#386

API clients could use a field on snippet objects to determine the type
of snippet they are looking at. Therefore, we change the snippet type to
an interface, similar to the page interface, so it can expose a new
field called `snippetType`.