Allow executemany to return rows

[LeonardBesson]

Fixes #448

This is a proposed change to support executemany with INSERT ... RETURNING.

The PR adds a flag parameter to executemany, but adding a seperate method like fetchmany was also mentioned.

@elprans would you consider something along those lines? I think the value here is allowing to take advantage of the query pipelining from executemany while avoiding the limitations of the proposed workarounds (like DEFAULT as mentioned in your stackoverflow post)

[elprans]

Plausible. I'm not a fan of changing the function signature based on the argument though. I think this should be a separate method, e.g. fetchmany or something like that (though could be somewhat confusing for DB-API users).

[elprans]

Ability to return values added as fetchmany in #1175.