Is it possible to set a StringMap() from environment?

If I define a StringMap() I can set multiple key/value pairs by repeating the flag:

```
--flag foo=bar --flag baz=bar
```

But this can not work with environment variables because you can only have one variable of the same name:

```
APP_FLAG=foo=bar
```

I could not find a way to set the second parameter with the env var - is this supported? I tried to add a \n to it but it didnt work.