Show correct line for template warnings, refs #102

As the line number is not readily exposed by Erubis::Basic::Converter
this determines line numbers by matching the regexp match offset against
lines within the file.

This seems a bit clunky but is probably preferable to copying and
modifying #convert_input.
FC007: Remove step duplication.
FC033: Remove step duplication.
FC033: Don't warn for hidden templates, refs #165.