Shorthand to write targets to a database

Maybe:
```r
...
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)
...
```



