Regexes

- Added regex support to substirutions.
- Fixed bug that prevented 1.9.4 from shipping with regexes.  If the
  description is "aXXaaXXa", and the substitution is /XX/.../ then the
  first substitutions changes the length of the string to "a...aaXXa"
  and therefore invalidates the index for the second match, and makes
  this change: "a...a...Xa".  The fix is to keep a running 'skew' count
  of the difference in 'from' and 'to' length, to adjust the match
  indexes.
- Moved the helper deltaSubstitutions function into the Task object,
  which makes more sense.
- Cleaned up output composition for CmdAdd.
- Eliminated #ifdef FEATURE_REGEX.  They are here to stay.

src/commands/Command.cpp

@@ -343,6 +343,16 @@ void Command::modify_task (Task& task, Arguments& arguments)
       description += arg->first;
     }
 
+    // Substitutions.
+    else if (arg->second == "subst")
+    {
+      std::string from;
+      std::string to;
+      bool global;
+      Arguments::extract_subst (arg->first, from, to, global);
+      task.substitute (from, to, global);
+    }
+
     // Any additional argument types are indicative of a failure in
     // Arguments::extract_modifications.
     else