Feature/better prompt support

extend the prompt capabilities to allow the below synthax:

```
{
    "project_name": "project-name",
    "project_key": "{project_name}",  # <- default value based on previous input
    "create_virtualenv" : {
        "default": "N",
        "type": "bool",
        "prompt": "Create virtualenv (`{project_name}`) [yN]"  # variable subtitution using .format() 
    },
    "year": {
        "default" : "2014",
        "type" : "int"
    }
}
```

this produce the following dictionary ( if default values accepted):
```
{  "project_name": "project-name",
    "project_key": "project-name",
    "create_virtualenv" : False,
    "year" : 2014
}
```