Options controlling the history
- enable '!' history expansion
- when exiting, append history entries to $HISTFILE, rather than replacing the old file; this is the default
- save additional info to $HISTFILE
- if the commandline starts with a whitespace, don't add it to history
- append every single command to $HISTFILE immediately after hitting ENTER.
- always import new commands from $HISTFILE (see 'inc_append_history' above)