Expressions

- Began Expression::toInfix to upgrade old-style filters to infix
  algebraic filters.
- Added operator support to Arguments::categorize.
- Modified CmdCustom.cpp as a read-only command guinea-pig for the
  new argument processing.

src/commands/CmdCustom.cpp

@@ -95,6 +95,10 @@ int CmdCustom::execute (std::string& output)
 ////////////////////////////////////
   Arguments f = context.args.extract_read_only_filter ();
   Expression e (f);
+  e.toInfix ();
+  e.toPostfix ();
+
+return 0;
   // TODO e.apply (tasks);
 ////////////////////////////////////