Add pluggable formatters for traceable lists
Add pluggable formatters for traceable lists

This commit contains the following changes:
- Re-factor the list processor to use flexible formatting classes,
  similar to those introduced for traceable matrices in commit
  78fcc92.
- Add a table formatter for traceable lists.
- Add support for specifying additional attributes of traceables
  to be included in the table format output.

t4ngo/sphinxcontrib-traceables