Thanks to @PlatinumPhoenix for the suggestion.

New command:
/salarysumup <filename> [-p or -g] - sums up the history of a .yml history file (per day or otherwise). Shows the totals for both players and groups (default) or only players ( -p option) or only groups ( -g option).

New entry to config.yml:
New entries to messages.yml:

Full change log:

• Refactoring of code
  • Code quality is now slightly better (though still pretty poor)
• Fixed issue with history file recording incorrect groups
  • Was an issue when get-all was set to true in the config
• Added name of group to history
  • When received group salary, the group name is now logged in the history
• Added per day logging
  • Currently only available in addition to regular history
  • Optional, disabled by default
  • Creates a separate file for each day
  • File names formatted per_day_history_<date>.yml
• Added command tab-completion
• Added the /salarysumup command
  • Can sum up history files
  • Lists per day history files in tab-completion
  • Also works for regular history files
    • Need to write the name manually
    • If you want to have these tab-completed as well, let me know
  • Shows the totals per day for each player and group
    • Options in command to only show player or group sums
  • Includes new messages