Add file_data decorator

This is a rework of #6 (courtesy of @bulkan).

The `file_data` decorator takes a filename as a parameter. The file will contain JSON data, which can be either a list (unnamed test cases) or a dictionary (named test cases, where keys will be used for representative names of the values to be shown in the output).

Examples:

```json
[
    "Hello",
    "Goodbye"
]
```

```json
{
    "unsorted_list": [ 10, 12, 15 ],
    "sorted_list": [ 15, 12, 50 ]
}
```