Load configuration from current path if there's no Play app.

There could be no Play app when it's for tests or console. At that time,
the same configuration still need to be provided to modules.

This patch changes BeyondConfiguration to find the configuration
from the current app path.
Fix encoding bug in ScriptableFileSystem.

The `string` and `write` methods of scalax.io's Resource implicitly take
a `Codec` instance, not a string.
Add test case for ScriptableFileSystem API.

In order to test the module, some assets are added.
Return undefined, if the field is optional and no values are set.

Currently, getting the value from optional field occurs error if value
is not set. It's a bug.

This patch makes it return an undefined.