Add the ability to load a remote environment values file (#1296)

Enables the user to specify a remote path for an environment values file, e.g.,

```yaml
environments:
  cluster-azure-us-west:
    values:
      - git::https://git.company.org/helmfiles/global/azure.yaml?ref=master
      - git::https://git.company.org/helmfiles/global/us-west.yaml?ref=master
  cluster-gcp-europe-west:
    values:
      - git::https://git.company.org/helmfiles/global/gcp.yaml?ref=master
      - git::https://git.company.org/helmfiles/global/europe-west.yaml?ref=master

releases:
  - ...
```

This is particularly useful when you co-locate helmfiles within your project repo but want to reuse the definitions in a global repo.

pkg/remote/remote.go

@@ -260,3 +260,15 @@ func (g *GoGetter) Get(wd, src, dst string) error {
 
 	return nil
 }
+
+func NewRemote(logger *zap.SugaredLogger, homeDir string, readFile func(string) ([]byte, error), dirExists func(string) bool, fileExists func(string) bool) *Remote {
+	remote := &Remote{
+		Logger:     logger,
+		Home:       homeDir,
+		Getter:     &GoGetter{Logger: logger},
+		ReadFile:   readFile,
+		DirExists:  dirExists,
+		FileExists: fileExists,
+	}
+	return remote
+}