Support supplying customization options via a configuration file

[cxreiff]

User should be able to create a ~/.config/ttysvr configuration file that has any of the options available via command line arguments, that take effect when ttysvr is called.

• Command line arguments should take precedence over configuration file.
• Configuration file parsing should be automatically generated from the Clap parser struct
  • Maybe using something like clap_serde_derive or twelf?
• Handle config file loading in a cross-platform way.