Inconsistency in how title_tmpl vs cat_col react to col_order omissions.

[kfogel]

Suppose column N is omitted from the col_order list in the run-time config file.

Right now, as of commit d131ad0, if N is the value of cat_col, an error will result, whereas if N is referred to in the value of title_tmpl, then it is used -- that is, column N from the original row, not from the reordered-and-filtered row, will be used in constructing the page title.

This seems inconsistent. If the column can still be used in the title, then it should still be usable for categories, or else it should be usable by neither (but I lean toward the former: let it be used in both).

This is not a showstopper problem, because a) users are unlikely to omit a column they're actually using in the title or as the category, and b) the behavior right now leads to a consistent result, even though the behavior itself is inconsistent.

(Issue ticket ported here from previous repository's ticket.)