Shorthand to write targets to a database

[daranzolin]

Maybe:

...
tar_plan(
  cars = mtcars,
  flowers = iris,
  tar_write_to_db(tables_loaded, cars, flowers)
  tar_sql(report, "inst/query.sql")
)
...

Then the db dependency could be specified in query.sql instead of individual tables?

-- tar_load(tables_loaded)
...