Display tab character

[valadaptive]

Currently, the tab character is not displayed at all--it has no width. Cosmic Text has "expanding tabs", which means they're aligned relative to the start of each row, but we don't need anything that fancy. I think the only option we need to expose is a "tab width" option, specified relative to the width of a single space character.

Not sure if this requires changes in Swash or if it can all be done within Parley.