Add configuration through a settings file

[Alphare]

This also introduces read-only mode and a way of switching off the alternate screen as discussed in #13.

[vamolessa]

I thought #7 was refering to a cli flag (like running verco --read-only for example).
By creating a global config for read only, you want to disable writing on repositories at all on a machine?

Also, I think we can just leave the alternate screen always there, so maybe we could wait a little bit until we add a config file.

[Alphare]

Well, I'd also like to configure whether status also shows summary (split them into two commands), because sum is much slower (for now) and I use it less frequently than status, also custom keybindings, whether to default to a command (I'd like Verco to open directly on the log), etc..
Those are just examples, but I didn't want to make a huge PR with enough to justify the settings.

I can add the cli flags if you want for --read-only/--no-read-only to overwrite the default value, but my use case is more of the "never write". I can see a setting that lets you pick which commands you want to use: I would use update, or evolve (I should make a PR), maybe histedit, but never merge, for example.

[vamolessa]

Will close this as a config file is beyond the scope of this project.