Add new defaults.database_prefix option

[slaFFik]

Now, that we have several sites_directories support, it might be a good idea to consider adding a new option under defaults called database_prefix.

All databases created under the same "userspace", and as directory and site name is controlled by the directory name you enter during the site creation process - it makes sense to also be able to differentiate between DBs using prefixes.

This will allow allow to prefix all work projects with the acme_ prefix, for creating example.test under a work directory will also prefix the DB (which is by default example) with a new prefix resulting in acme_example.

What do you think about this?