`pgdo something …` should expand to `pgdo exec something …`

[allenap]

Already a bare pgdo command will expand to pgdo shell. In practice I find myself wanting to type pgdo a_thing to run a_thing in configured PostgreSQL environment – I keep forgetting to type exec. It would be cool if pgdo could retain its present behaviour, but when extra arguments are provided they should elicit an exec-like behaviour. Then it might be worth removing pgdo shell in favour of pgdo psql, perhaps pgdo exec too.