Add support for auto-expansion of application custom data

[the-overengineer]

Adds a configuration option to config.web to expand the stormpathApplication customData field as:

web:
  application:
    expand:
      customData: true # defaults to false to keep backwards compat

If specified, the expansion will automatically happen when attaching the stormpathApplication property to req.app. Do note, however, that the setting of stormpathApplication happens only once, on application load, so any updates will still need to be saved manually.

Fixes #440