- Updated for IDL V6.1.

- Find additional linked keyword sections, by setting add_keywords, even
  when scanning Keywords Sections (not just syntax keywords).
- Allow linking keywords from routines for which we don't know the type
  (fun or pro).
- Can specify "only keys", separate from "has" for add_keywords (has
  only works if they show up in the Syntax section).
- Trim multi-line HTML tags.
- Leading multi-plexers like [XYZ], in addition to {X|Y|Z} have shown
  up.
- Allow parse callbacks to end processing for that section by returning
  -1.
- Remove space around underscores.
- Various new special matchers, lowercase keywords, a bogus DATAMINER
  match against Syntax, etc.