annotate: automatically populate fields referenced from template If '{line_number}' is in the template, we'll probably need it. No --line-number option should be required in such cases.

annotate: reverse mapping between option name and field name This makes the next patch slightly simpler.

formatter: provide hint of referenced field names I don't like the function name, but I call it datahint() for consistency with another function I'll add later, fm.contexthint().