Change the way `option` param works

[francisbarton]

Instead of supplying option = 1 as default, use NULL as default.
Then if user supplies option, we can dispense with the feedback about which dataset is being used for the lookup/bounds. And we can still use if/else to supply option 1 as a default, with the feedback, if option remains NULL.
This will help make output less messy when user is sure which option they want (a bit like readr::read_csv() is quiet if you tell it what col_types to expect, but gives you a load of feedback about what it's doing, if you don't).