Allow backslash line continuations in pgbench's meta commands.

Allow backslash line continuations in pgbench's meta commands.

A pgbench meta command can now be continued onto additional line(s) of a
script file by writing backslash-return.  The continuation marker is
equivalent to white space in that it separates tokens.

Eventually it'd be nice to have the same thing in psql, but that will
be a much larger project.

Fabien Coelho, reviewed by Rafia Sabih

Discussion: https://postgr.es/m/alpine.DEB.2.20.1610031049310.19411@lancre