[config] add config:set --encoded (base64)

You can now set b64encoded values. They are
stored decoded, this is so dokku over ssh can set
odd config values
```
config:set --encoded KEY=BASE_64_VALUE
```

plugins/config/src/subcommands/set/set.go

@@ -1,11 +1,11 @@
 package main
 
 import (
+	"encoding/base64"
 	"flag"
-
-	"strings"
-
+	"fmt"
 	"os"
+	"strings"
 
 	common "github.com/dokku/dokku/plugins/common"
 	"github.com/dokku/dokku/plugins/config/src/configenv"
@@ -15,6 +15,7 @@ import (
 func main() {
 	args := flag.NewFlagSet("config:set", flag.ExitOnError)
 	global := args.Bool("global", false, "--global: use the global environment")
+	encoded := args.Bool("encoded", false, "--encoded: interpret VALUEs as base64")
 	noRestart := args.Bool("no-restart", false, "--no-restart: no restart")
 	args.Parse(os.Args[2:])
 	shouldRestart := !*global && !*noRestart
@@ -43,7 +44,15 @@ func main() {
 		if len(parts) == 1 {
 			common.LogFail("Invalid env pair: " + e)
 		}
-		env.Set(parts[0], parts[1])
+		key, value := parts[0], parts[1]
+		if *encoded {
+			decoded, err := base64.StdEncoding.DecodeString(value)
+			if err != nil {
+				common.LogFail(fmt.Sprintf("%s for key '%s'", err.Error(), key))
+			}
+			value = string(decoded)
+		}
+		env.Set(key, value)
 		changed = true
 	}