add a simple config file parser

Adds a simple config file parser. Config files are JSON files (or js files that export a single object) of the following format:

```javascript
{
    "generator-plugin-name-1" : {
        // options passed to generator-plugin-name-1 when loaded
    },
    "generator-plugin-name-2" : {
        // options passed to generator-plugin-name-2 when loaded
    },   
}
```

Config files can be in either the user's home directory or the current working directory. They can have the name "generator.json", "generator.js", ".generator.json", or ".generator.js". All found config files are merged. Files in the current working directory take precedence.

The config options for a plugin are passed as a second argument to the ```init``` call