Adds support for hex and rgb values in config.

[samuelstevens]

• improves support for #24 (hex).

Design

• Adds a struct to represent the config right after loading from the .ini file using primitive types (string, int, bool, etc.)
• Adds a struct to represent the config with rich types (tcell.Color).
• Prints any errors in the configuration to the console.
• config.Color is a simple wrapper around tcell.Color that has a .HexCode method to return a "#RRGGBB" string where needed.