Allow file-specific parts in .eslintrc

eslint allows you to override settings in a subdirectory, so that tests have a different configuration from other files.

This PR allows you to specify different settings for different files in the same directory. 

For example:
```
    {
        "globals": {
          //...
        },
        "rules": {
          //...
        },
        "fileSpecificConfigs": {
            "*.test.js": {
                "globals": {
                    "describe": true,
                    "it": true,
                    "expect": true
                }
            }
        }
    }
```