Fix #112; add results.Opt support

[nitely]

Fix #112

• Use results throughout; except for config_files internals which require toml support, pending Add results support nim-toml-serialization#100
• Add Opt[T] support for option types.

Alternatively it could be moved to a confutils/pkg/results, but since it's used internally and would need to be exported anyway, I think there is no point in that.