Two new functions `_bash-it-history-auto-save()` and `_bash-it-history-auto-load()`, which append new history to disk and load new history from disk, respectively.
See bash-it/bash-it#1595 for discussion.
- remove the use of redundant cite
- minor cleanups to history.plugin.bash
- set explicit load orders for history-search and history-substring-search
- add new plugin history-eternal
- replace superfluous trap for managing HISTTIMEFORMAT changes