from #83

The only thing that may cause confusion is that text-offset is in ems, while icon-offset is in pixels. But maybe that's not important.

The current units for icon-offset aren't even exactly pixels. If the icon gets scaled with icon-size then the offset also gets scaled. An icon with "icon-offset": 6 and "icon-size": 0.5 actually has an offset of 3 pixels.

For now the units need to be relative to the unscaled size, not absolute values, but maybe we should changes the units of icon-offset to a fraction of the original size.