[#493] cmd/node: Implement a basic configuration component

Create `config` package nearby storage node application. Implement `Config`
as a wrapper over `viper.Viper` that provides the minimum functionality
required by the application.

The constructor allows you to read the config from the file. Methods are
provided for reading subsections and values from the config tree. Helper
functions are implemented to cast a value to native Go types.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/config/calls.go

@@ -0,0 +1,28 @@
+package config
+
+import (
+	"github.com/spf13/viper"
+)
+
+// Sub returns sub-section of the Config by name.
+func (x *Config) Sub(name string) *Config {
+	return (*Config)(
+		(*viper.Viper)(x).Sub(name),
+	)
+}
+
+// Value returns configuration value by name.
+//
+// Result can be casted to a particular type
+// via corresponding function (e.g. StringSlice).
+// Note: casting via Go `.()` operator is not
+// recommended.
+//
+// Returns nil if config is nil.
+func (x *Config) Value(name string) interface{} {
+	if x != nil {
+		return (*viper.Viper)(x).Get(name)
+	}
+
+	return nil
+}