Automatic cache updates

[dbrgn]

As discussed in #73, there could be a config option where - if it's set - the cache automatically refreshes itself when it's outdated.

Implementation notes (copied from here):

This would require a slight reorganization of the main function:

• Handle args.flag_config_path and args.flag_seed_config right after handling args.flag_version (simply move up the code blocks)
• Move the config loading code in print_page into the main function, right after the args.flag_seed_config handling block
• In src/config.rs, add the new config elements to the RawConfig and Config structs and add the defaults to RawConfig::new().

The default configuration can be generated with tldr --seed-config. My suggestion would be:

[updates]
auto_update = false
auto_update_interval_hours = 720

(The auto_update_interval_hours could then be set to MAX_CACHE_AGE * 24 by default.)