feat(helm): add --password-stdin to `helm repo add`

**What*

Without this commit, you can't pipe the password into `helm repo add`:

```
$ echo password | helm repo add repo-name https://repo-url --username username
Password:
Error: inappropriate ioctl for device
```

This commit introduces `--password-stdin`:

```
$ echo password | helm repo add repo-name https://repo-url --username username --password-stdin
"repo-name" has been added to your repositories
```

**Why**

There are two reasons I see for adding this:

* I personally would expect that it's possible to pipe the password into
  `helm repo add` but that's currently not the case. If I understand it
  correctly, you currently either need to pass the password via a cli
  parameter (`--password`) or use a expect/send mechanism.

* Subcommands like `helm registry login` already support
  `--password-stdin`. The cli interfaces should be consistent regarding
  which options they support.

**Notes**

I basically just copy-pasted code from `cmd/helm/registry_login.go`.

Signed-off-by: André Schröder <andre.schroedr@gmail.com>

cmd/helm/repo_add.go

@@ -48,6 +48,7 @@ type repoAddOptions struct {
 	url                  string
 	username             string
 	password             string
+	passwordFromStdinOpt bool
 	passCredentialsAll   bool
 	forceUpdate          bool
 	allowDeprecatedRepos bool
@@ -85,6 +86,7 @@ func newRepoAddCmd(out io.Writer) *cobra.Command {
 	f := cmd.Flags()
 	f.StringVar(&o.username, "username", "", "chart repository username")
 	f.StringVar(&o.password, "password", "", "chart repository password")
+	f.BoolVarP(&o.passwordFromStdinOpt, "password-stdin", "", false, "read chart repository password from stdin")
 	f.BoolVar(&o.forceUpdate, "force-update", false, "replace (overwrite) the repo if it already exists")
 	f.BoolVar(&o.deprecatedNoUpdate, "no-update", false, "Ignored. Formerly, it would disabled forced updates. It is deprecated by force-update.")
 	f.StringVar(&o.certFile, "cert-file", "", "identify HTTPS client using this SSL certificate file")
@@ -143,14 +145,24 @@ func (o *repoAddOptions) run(out io.Writer) error {
 	}
 
 	if o.username != "" && o.password == "" {
-		fd := int(os.Stdin.Fd())
-		fmt.Fprint(out, "Password: ")
-		password, err := term.ReadPassword(fd)
-		fmt.Fprintln(out)
-		if err != nil {
-			return err
+		if o.passwordFromStdinOpt {
+			passwordFromStdin, err := ioutil.ReadAll(os.Stdin)
+			if err != nil {
+				return err
+			}
+			password := strings.TrimSuffix(string(passwordFromStdin), "\n")
+			password = strings.TrimSuffix(password, "\r")
+			o.password = password
+		} else {
+			fd := int(os.Stdin.Fd())
+			fmt.Fprint(out, "Password: ")
+			password, err := term.ReadPassword(fd)
+			fmt.Fprintln(out)
+			if err != nil {
+				return err
+			}
+			o.password = string(password)
 		}
-		o.password = string(password)
 	}
 
 	c := repo.Entry{