replaced viper with koanf, feature parity

[woutslakhorst]

closes #4

This PR focusses on removing viper and the reflection for config loading. It does not focus on refactoring engines and load/config ordering.

[codecov]

Codecov Report

Merging #44 (2e7478d) into master (60c38b5) will decrease coverage by 0.14%.
The diff coverage is 92.63%.

Impacted Files	Coverage Δ
network/config.go	100.00% <ø> (ø)
vdr/config.go	0.00% <ø> (ø)
core/engine.go	86.95% <77.77%> (-2.52%)	⬇️
cmd/root.go	77.27% <85.71%> (+3.83%)	⬆️
core/config.go	93.87% <93.47%> (+6.69%)	⬆️
core/status.go	100.00% <100.00%> (+10.34%)	⬆️
crypto/engine/engine.go	80.00% <100.00%> (ø)
network/engine/engine.go	77.64% <100.00%> (+0.53%)	⬆️
vdr/engine/engine.go	82.95% <100.00%> (+0.39%)	⬆️

[woutslakhorst]

drop in coverage is due to removing code....