Integrate SpanMarker library (#737)
* Integrate SpanMarker library
* Add missing SpanMarker docs
* Use 'span-marker' instead of 'span_marker' as lib key
1. This is more in line with the other libraries, and
2. my codebase has been using 'span-marker' from the start.
If I stick with span_marker, then all models will get the 'span_marker' AND 'span-marker' tags.
* Update the concise docs
