Add support for custom filters

[dmolsen]

This feature would work in a similar way to the Mustache Helper feature. Twig helpers could be installed via composer and they'd register themselves. Currently this is how the Mustache helper works:

• install helper which gets registered
• on load the helper is associated with a dispatcher event
• when the pattern engine loads it fires the event and all helpers register themselves

Example mustache helper.

[dmolsen]

The extension points have been added. Need to add the install feature to InstallerUtil in pattern-lab/core and update the types in pattern-lab/unified-asset-installer.

[dmolsen]

Done.