Drop local database before pulling stage DB

To help promote sanity when debugging "tables existing" errors.

Note: This will prompt you "Are you sure?".  I'm into leaving this warning in, to further notify a developer "Hey you're' about to lose any of your DB work" ... but, could be open to adding the `-y` option in.

````
vagrant@dev:/var/www/vagrant$ bin/ds pull stage --db
Dropping local DB
Do you really want to drop all tables in the database dosomething? (y/n): y
Pulling down db from staging
````
It's better to change this to `drush -y sql-drop`.