Support navigation

[xleclercq]

Suggested format:

{
"title": "Test Site 1",
"homepage": { "page": "pages/index.html" }, // TODO: has to refer to existing content defined below
"navigation": [
{
"label": "Home",
"target": { "page": "pages/index.html" }
}
],
"content": [
"page": { "path": "pages/index.html" }
]
}