Provide connivence accessors on SourceMap's for begin/end line/column

[kylef]

I am sure we can make a more intutive syntax for this, here's exerp from the Paw API Blueprint importer:

      parseResult.errors.forEach((error) => {
        const start = error.attributes.get('sourceMap').get(0).get(0).get(0);
        const line = start.attributes.get('line');
        const column = start.attributes.get('column');
        throw new Error(`${error.toValue()} on line ${line}:${column}`)
    });