Handle nested keys

It'd be neat if toiletdb methods that accepted key parameters handled nested keys. For example:

// db.json = { ports: { http: 80, https: 443 } }
db.read('ports.http', function(err, data) {
  console.log(data)
  > 80
})

or:

// db.json = { ports: { http: 80, https: 443 } }
db.read(['ports', 'http'], function(err, data) {
  console.log(data)
  > 80
})

The second method avoids confusion where '.' may be a part of an attribute's name, ex: data['example.org']

Start by locating the toiletdb methods that accept key parameters, especially db.read, and inspect how they currently resolve keys from the JSON backend. Done means nested keys work with both dot-separated strings such as 'ports.http' and key arrays such as ['ports', 'http'], while preserving support for attribute names containing dots.