Add unit tests

[kylekatarnls]

Hi, here are some proposals for your engine:

• Add unit tests to ensure all works as expected and will continue to
• Travis CI configuration to get each commit tested automatically on all supported versions of PHP: https://travis-ci.org/pug-php/pug-slim
• Use DIRECTORY_SEPARATOR and trim template path to get cross-os compatibility

[MarcelloDuarte]

🍻