API: Add rawcontinue parameter to action=query

Soon the simplified continuation will become the default for
action=query. This parameter will then serve select the return of the
raw continuation data. To allow for clients to transition without
"unrecognized parameter" warnings, I'd like to add the parameter now
before even adding deprecation warnings.

Change-Id: I5f6895b6a94c53a231a50707ee9463f91295da8f
Merge "API: Add rawcontinue parameter to action=query"