Ion Variables Patch

Several changes were necessary to allow Ion to use variables for tuning the shell. The History structure, for example, no longer needs to have a max_size variable because this information is already stored as a variable in the shell. This patch adds a few system variables such as CWD, HISTORY_FILE, HISTORY_FILE_ENABLED, HISTORY_SIZE, and DIRECTORY_STACK_SIZE. It also adds the capability for history being logged to a history file, named ion_history. What's currently lacking from this implementation is limiting the size of the history log and not logging commands loaded from files in history.