Add data/ directory support for apps

This commit introduces data/ functionality to protos applications.

Any JSON files located inside data/ will be automatically read and
stored into the application's data object.

For example, parsed contents of the data/test.json can be read by
accessing app.data('test'). All JSON files within data/ will be parsed.

A new protos command has been implemented: `protos data`, which
automatically generates JSONfiles into the data/directory. The extension
can be omitted and will be assumed as JSON.

Only JSON files will be read from the data/ directory.