Simple EditorConfig parser with error reporting and tests

- Defined class EditorConfig to parse and validate EditorConfig content.
- Added tests to check for correct values and extra options

No actual glob logic attached at present.

@devoncarew @gaurave 
Once this has access to the file system (using the [files package](https://github.com/google/files.dart)?), I think the ideal API would be to just pass a file path, and it then climbs the file system merging .editorconfig's until it finds a root one, and then returns a simple value object which exposes all the properties, and the most logicial name for *that* class would be `EditorConfig`.  Maybe rename this class to `EditorConfigFile` or similar?