Expand label schema to allow more than a single string

[kpreid]

Possible use cases:

• Nested progress labels (e.g. "directory /foo, file bar.json, 100 of 2438 bytes") without requiring the caller to allocate concatenated strings or to be aware of the entire hierarchy.
• Machine-readable data referring to application-specific objects, so the progress rendering can have icons/links/hover/rich-text features
• Accumulate a count of successful or failed items.