Doesn't pull configuration from `auth.local` by default

[mcchrish]

Steps to reproduce

App:

app.configure(authentication(app.get('auth')))
  .configure(local())

default.json:

{
  auth: {
    local: {
      usernameField: 'username'
    }
  }
}

The above throws a missing credentials error when trying to authenticate.

Expected behavior

local() should automatically get the local auth config in the configuration file. By default, it's auth.local.

Actual behavior

It didn't get the local auth config.

Work around:

app.configure(app.get('auth').local)

System configuration

Tell us about the applicable parts of your setup.

Module versions (especially the part that's not working): 0.2.0 NodeJS version: 7.1.0 Operating System: macOS Sierra Browser Version: Firefox 50