Add help function for dropping a table

[carlhiggs]

Is your feature request related to a problem? Please describe.
There is currently a drop() function within the ghsci.Region object, but this drops a database. Sometime you just want to drop a table.

Describe the solution you'd like
If a user supplies no argument, proceed to attempt to drop database if the user confirms they wish to do this.

However, if a word is supplied, then that should be assumed to be a table and an attempt made to drop this.

Additional context
This will be useful for developing / testing new functionality, but also for advanced users who use the region object subfunctions as convenient helps for loading their own data to a region's spatial database. If they do that, there should also be the easy option to get rid of it. We don't need a new function to do this, just introduce polymorphism into the existing one.