essync configuration parameters.

Added a -settings and a -mapping flag to essync to
allow the user to configure the elasticsearch index correctly
before syncing the data. This saves manually setting up the server.
FWIW when there are no required side effects, and all else being equal,
I'd prefer to keep err variables local. It means there's less to look
for when scanning the code. In this case (and others further down)
I'd probably use the "if err := ...; err != nil {" idiom.