Missing API to edit database permissions

Nano does not have any functionality to add permissions (users/roles) to a database. Always needed after DB creation.

expected - an API like:
```
nano.db.security([callback]) -- getting the security info
nano.db.secure(permObj, [callback]) -- setting security object
```

reason:
Creating a user and manually its database (to know when it is ready to create multiple documents inside), there is no owning user set.
`db.create(name, callback)` has no option to set user security.
Other basic functions also have a wrapper (like `.get()`).
Would help with making it easy to use couchDB without having to read and abstract the couchdb-docu-curl calls to a `nano.request`.

workaround - call manually:
```
// retrive
let perms = await nano.request({db: targetedDb, method: 'get', path: '/_security');

// save
await nano.request({db: targetedDb, method: 'put', path: '/_security', body:
{
admins: { names: [newuser_email, 'Administrator'], roles: ["admins"] },
members: { names: [newuser_email], roles: [] },
}
});
```

Start by tracing nano.request and the existing nano.db wrapper methods to understand how database-scoped APIs are exposed. Compare the proposed security and secure operations with the supplied _security GET and PUT workaround; done means callers can retrieve and update database permissions without constructing those requests themselves.

Written by the indexing model from the issue text.