Allow subcommand flattening

There is already an issue open for struct flattening. This one is specifically for subcommand flattenin.
Here is use-case example (using proc-macro from second comment):

```rust
#[derive(FromArgs, PartialEq, Debug)]
/// Top-level command.
struct TopLevel {
#[argh(subcommand)]
nested: MySubCommandEnum,
}

#[derive(PartialEq, Debug, FlattenSubcommand)]
enum MySubCommandEnum {
Global(MySubCommandEnumGlobal),
Sys(MySubCommandEnumSys),
}

#[derive(FromArgs, PartialEq, Debug)]
#[argh(subcommand)]
enum MySubCommandEnumGlobal {
One(SubCommandOne),
Two(SubCommandTwo),
}

#[derive(FromArgs, PartialEq, Debug)]
#[argh(subcommand)]
enum MySubCommandEnumSys {
Three(SubCommandThree),
Four(SubCommandFour),
}

#[derive(FromArgs, PartialEq, Debug)]
/// First subcommand.
#[argh(subcommand, name = "one")]
struct SubCommandOne {}

#[derive(FromArgs, PartialEq, Debug)]
/// Second subcommand.
#[argh(subcommand, name = "two")]
struct SubCommandTwo {
#[argh(option)]
/// x
x: usize,
}

#[derive(FromArgs, PartialEq, Debug)]
/// Third subcommand.
#[argh(subcommand, name = "three")]
struct SubCommandThree {}

#[derive(FromArgs, PartialEq, Debug)]
/// Fourth subcommand.
#[argh(subcommand, name = "four")]
struct SubCommandFour {}

fn main() {
let up: TopLevel = argh::from_env();
println!("{:?}", up);
}
```