Argument Parsing

- Implemented Arguments::extract_read_only_filter to isolate the
  arguments that pertain to read-only command filters
- Implemented Arguments::extract_write_filter to isolate the arguments
  that pertain to write command filters.
- Implemented Arguments::extract_modifications to isolate the arguments
  that pertain to write command modifications.
- Created stubbed Expression object.
- Began integration of Expression and Arguments into commands/CmdCustom.

src/CMakeLists.txt

@@ -15,6 +15,7 @@ set (task_SRCS API.cpp API.h
                Date.cpp Date.h
                Directory.cpp Directory.h
                Duration.cpp Duration.h
+               Expression.cpp Expression.h
                File.cpp File.h
                Filter.cpp Filter.h
                Hooks.cpp Hooks.h