First integration for mock data files

- YAML
- JSON
Add mock support for Jekyll-style headers

- variables can now be defined in YAML at the top of a page
- add tests for inner-page mock
Engines now use .extension format

- each extension must begin with a “.”
- increases clarity while scanning files
Update README
Private Files

- hide any file starting with _
- make sure bundles do not concatenate files with an _
- rename all data files to prefix with _
- private template files should also start with _
Bump to 0.0.4