Enhancement - related to, but not fixing bug #293

- Added new attribute modifiers 'word' and 'noword' which find the existence
  of whole words, or prove the non-existence of whole words.  If a task has
  the description "Pay the bill", then "description.word:the" will match, but
  "description.word:th" will not.  For partial word matches, there is still
  "description.contains:th".
- Added unit tests for the text.cpp functions.
- Added unit tests including the new modifiers in filters.
- Added unit tests to parse the new modifiers.
- Modified man page.
- Modified the Context::autoFilter processing to use the new modifiers for
  +tag and -tag filtering.
- Added a support email to an error message, while looking at the filter code.
- Added new modifiers to the help report.
- Modified a utf8.t unit test to include an alphanumeric tag, rather than a
  smiley face.

src/Context.cpp

@@ -690,6 +690,9 @@ void Context::parse (
 }
 
 ////////////////////////////////////////////////////////////////////////////////
+// Note: The reason some of these are commented out is because the ::clear
+// method is not really "clear" but "clear_some".  Some members do not need to
+// be initialized.  That makes this method something of a misnomer.  So be it.
 void Context::clear ()
 {
 //  Config                    config;
@@ -759,6 +762,7 @@ void Context::autoFilter (Task& t, Filter& f)
     }
 
     // The mechanism for filtering on tags is +/-<tag>.
+    // Do not handle here - see below.
     else if (att->second.name () == "tags")
     {
     }
@@ -776,17 +780,22 @@ void Context::autoFilter (Task& t, Filter& f)
     }
   }
 
+  // This is now a correct implementation of a filter on the presence or absence
+  // of a tag.  The prior code provided the illusion of leftmost partial tag
+  // matches, but was really using the 'contains' and 'nocontains' attribute
+  // modifiers.  See bug #293.
+
   // Include tagAdditions.
   foreach (tag, tagAdditions)
   {
-    f.push_back (Att ("tags", "has", *tag));
+    f.push_back (Att ("tags", "word", *tag));
     debug ("auto filter: +" + *tag);
   }
 
   // Include tagRemovals.
   foreach (tag, tagRemovals)
   {
-    f.push_back (Att ("tags", "hasnt", *tag));
+    f.push_back (Att ("tags", "noword", *tag));
     debug ("auto filter: -" + *tag);
   }
 }