Support custom engines

[allejo]

Add support for different template engines and rendering engines. Examples:

• Nette
• PHP's CommonMark

This will be implemented by allowing registration of new engines based on file extension. Each engine must implement an interface that will be used by stakx internally.

[allejo]

Updates to this issue:

• The template engine (49dec4c) and markup engines (Move markup engines to a DI setup in the container #61) have been abstracted out and interfaces have been introduced.
• stakx has internal functionality to register markup extensions with our container

What's left to do:

• Add functionality to be able to register template engines based on extensions
• Expose functionality to register markup engines and extensions