Re: stylelint/stylelint#133

Right now the positions reported by errors or warnings rely on the node option; therefore, the position is approximate but not exact, since a node might contain any number of lines, and the warning/error might not relate to the first line.

To increase accuracy, a few things could be done:

• Add an index option to the functions (alongside node), which would allow us to register an exact index for the item relative to the specified node. PostCSS would then calculate a line/column position based on the node's raw string value and this reported index.
• Add line and column options, so that whoever is registering the warning/error can dictate exactly what position gets reported.

Any other ideas?