chore(nunjucks/mockPackage): add mockPackage for testing
chore(package.json): update to marked v3.x

This gives us access to the marked `Renderer` class for better customization
of the rendering process.
feat(nunjucks/renderMarkdown): add new service to allow custom markdown rendering
fix(nunjucks/marked custom tag): use renderMarkdown service

This will trim indentation from code before rendering, which fixes
https://github.com/angular/angular.js/pull/8963
fix(nunjucks/marked filter): use renderMarkdown service

The service will trim indentation from code before rendering, which fixes
https://github.com/angular/angular.js/pull/8963