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:

• cache_policy
• image_preview.lazy_load
  • That option is used inconsistently anyway and should probably be removed
• state_event_display
  • need to change the logic to store state events that are not displayed
• mouse, terminal.enable_extended_keys
  • need to modify the panic hook
• improve completions and argument parsing
  • easier after Improve completions #520
• add tests

Unsupported options:

• default_room
  • doesn't make sense to change at runtime
• image_preview.protocol.font_size
  • option is deprecated
• terminal.enable_title
  • I think there is no way to restore old value
• max_log_files, request_timeout, ssl_verify, proxy
  • hard/impossible to change at runtime

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

fixes #505
supersedes #510
fixes #367
fixes #177

[VAWVAW]

I believe this is useful in its current state and the missing settings can be added in follow up PRs.