Simplify configuration loading process (#15)

The process of loading configs is very convoluted right now: we have interpolated files, non-interpolated files, dynamic providers, functions to look for environment...

We can greatly simplify this process by having a single function that will load all the requested YAML files.

command_line_provider.go

@@ -24,7 +24,7 @@ import (
 	"fmt"
 	"strings"
 
-	flag "github.com/ogier/pflag"
+	flag "github.com/spf13/pflag"
 )
 
 // StringSlice is an alias to string slice, that is used to read comma separated flag values.
@@ -43,6 +43,10 @@ func (s *StringSlice) Set(val string) error {
 	return nil
 }
 
+func (s *StringSlice) Type() string {
+	return "StringSlice"
+}
+
 type commandLineProvider struct {
 	Provider
 }

command_line_provider_test.go

@@ -23,7 +23,7 @@ package config
 import (
 	"testing"
 
-	flag "github.com/ogier/pflag"
+	flag "github.com/spf13/pflag"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )