Add basic support for interceptors

@JakeWharton @jhump @swankjesse 
Completely off topic: this spelling led me to grammarist:

> Okay, OK, and O.K. are all acceptable spellings of the word. OK is more common in edited writing, but okay appears about a third of the time. O.K. is preferred by a few publications, including the New York Times, even though it is not an abbreviation of anything in modern use.