Add commands :set and :reload

[VAWVAW]

Allow the config to be reloaded and specific settings to be changed at runtime.

Most config values under settings are changeable and dirs.downloads is updated on :reload.

todo:

• state_event_display
  • need to change the logic to store state events that are not displayed
  • trivial to implement after Save all edit events internally #445
• mouse
  • this option is not documented btw
  • need to modify the panic hook
• notifications
  • probably need global state to save the settings
• image_preview
  • would be easier to implement after Render image previews lazily #464
• users
  • how do we express this as a :set option?
• sort
  • probably like :set sort.rooms = name,alias
• improve completions and argument parsing
  • easier after Improve completions #520
• add tests

Macros should be added in a followup that adds a :map command.

fixes #505
supersedes #510