Support configuration files - At least for output formatting options (colors)

[rainerschoe]

Describe the solution you'd like
It would be nice to have a way to persistently configure default behavior of the tool:

• Color-Classes for output formatting
• Timeouts
• Greeting message at start of program
• Default TCP port
• Grammar caching enable/disable
• Certs / Keys for secure channels

This configuration obviously needs to be stored on the file-system.
If no configuration is found, default values should be used.

Describe alternatives you've considered
specify all config options on command-line. But this is really annoying, as it forces the user to implement wrappers.

Describe why you think this feature is useful for gWhisper users
Especially for colors this is important, as the used color-set depends on the selected background color of the users terminal. Black on black or white on white does not make sense.

[rainerschoe]

this might also be a prereq for secure channel support #6 as we need some way to store credentials.

[rainerschoe]

Idea: we could make CLI Options and config files basically the same thing.
I.e. The config file is parsed with a sub-graph of the grammar and then added to the parse-tree of the options grammar.