Generate pages on the fly from data

**Current State**
Put HTML and markdown into .html files stored in /pages and you get a url per page:

```
/pages
..../home.html
..../contactus.html
..../info1.html
/layouts
..../default.html
/data
....

$ foundation build

// resulting URLs
http://example.com/home.html
http://example.com/contactus.html
http://example.com/info1.html
```

**Requested Capability**
Put HTML and markdown into a hierarchical JSON doc get a url per page:

```
/pages
..../empty
/layouts
..../default.html
/data
....pages.json (this is a 10MB JSON file with a record per page)

$ foundation build

// resulting URLs
http://example.com/home.html
http://example.com/contactus.html
http://example.com/info1.html
```

I have a use-case for this. My project has a SQL database and we'd like to create a static, read-only site where there is 1 page per record. I can write a program to create the json file from the database and I can also generate a whole bunch of /pages/*.html files from the database. In current state, I will have to generate a whole bunch of /pages/*.html files.

**Potential API:**
The root parameter can take a directory for physical .html files and/or a hierarchical JSON doc.

```
var jsonDocOfPagesData =

/// ...

.pipe(panini({
root: ['src/pages/', jsonDocOfPagesData],
layouts: 'src/layouts/',
partials: 'src/partials/',
data: 'src/data/',
helpers: 'src/helpers/'
}))
```