Filter Bugs

- A substitution arg is categorized as "subst", but was tested for
  "substitution".
- The Arguments::is_multipart method was triggering on '/', which is
  wrong, but I can't remember why I put it in there.  Hmmm.
- When extracting a write-filter, substitutions after the command
  were generating an error, whereas they should be ignored.
- Task::substitute was modifying data even when the patterns were not
  matching.  Yikes.
- Added unit tests for en-passant modification of all types during
  done, delete, start, stop.

src/Context.cpp

@@ -119,8 +119,7 @@ int Context::initialize (int argc, const char** argv)
     // Instantiate built-in column objects.
     Column::factory (columns);
 
-    // Categorize all arguments one more time.
-    // TODO This may not be necessary.
+    // Categorize all arguments one more time.  THIS IS NECESSARY.
     args.categorize ();
 
     // Handle default command and assumed 'info' command.