Support reading mustache 'lists' from files

This commit add support for reading mustache 'lists' from files, so you
can use the list section functionality when templating things.

An example of the list syntax is as follows:

```
{package_commands, {list, [[{name, "riak"}], [{name, "riak-admin"}], [{name, "search-cmd"}]]}}.
```

Then you can, for each of the list elements, render some text:

```
{{#package_commands}}
chmod +x bin/{{name}}
{{/package_commands}}
```

As far as I can tell, previously this was impossible, as everything read from a file was treated as a single value, not a list.