Option<Vec<T> as argument

Hi,

An argument with Option<Vec<T>> does not seem to work. Are there any workarounds ?
```
#[derive(StructOpt, Debug)]
pub enum DevCmds {
    #[structopt(name = "list", alias = "show")]
    Cmd1 {
        optional_args: Option<Vec<String>>,
    },
```

This produces errors like:
```
error[E0277]: the trait bound `std::vec::Vec<T>: std::str::FromStr` is not satisfied
```