Adds writeDisposition configuration property

Adds ability to define the writeDisposition strategy for query insert jobs.
more info  - https://cloud.google.com/bigquery/docs/tables
i might actually vote to leave the default as `NULL`, but tweak the logic below to only include `writeDisposition` in the body if the value isn't `NULL`. this way we're not responsible for knowing the default, the server is.

wdyt @hadley ?