[#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/opts.go

@@ -0,0 +1,20 @@
+package config
+
+type opts struct {
+	path string
+}
+
+func defaultOpts() *opts {
+	return new(opts)
+}
+
+// Option allows to set optional parameter of the Config.
+type Option func(*opts)
+
+// WithConfigFile returns option to set system path
+// to the configuration file.
+func WithConfigFile(path string) Option {
+	return func(o *opts) {
+		o.path = path
+	}
+}