Add a `stroke-alignment` property

[Jesus89]

As discussed in this issue: https://github.com/CartoDB/renderer-prototype/issues/184, it would be nice to have this new property to align the stroke. The possible values are: center (that is the default behavior), inside and outside.

More info: https://css-tricks.com/text-stroke-stuck-middle/

[makella]

@Jesus89 @davidmanzanares we should revisit this now that we have placement options for points and lines. In addition, we will want the ability to offset strokes of both polygons and points.