Initial implementation of config parser

[pacak]

Implements #275

• - NamedArg::key gives an easy "read this thing from the config" method
• - Parser::key behaves similar to Parser::fallback and gives "if not specified - try to use config"
• - how to pass config to the parser? Ended up implementing OptionParser::config_parser
  • want to be able to run tests or custom runs - use with OptionParser::run_inner
  • want to be able to specify config file name on a command line
• - strictly typed config reader in addition to stringly typed? Generic parsers tend to produce an enum Value that can be one of String, f64, etc. should Config::get return a local version of that?
• - how should a single key annotation on a field behave?
  • it can behave similarly to env and add a fallback to the config
  • it can behave like short/long and override the behavior making it easy to create pure config reader
• - API documentation
• - tutorial documentation
• - instances of Config for a few formats? yaml/json/toml?
• - simple "ini" config reader
• - derive macro support