Inconsistent help-related behavior around subcommands

Consider a basic minimal bit of argh code centered on subcommands, something like this:

```rust
use argh::FromArgs;

#[derive(FromArgs)]
/// Example
struct Args {
#[argh(subcommand)]
subcommand: Subcommand,
}

#[derive(FromArgs)]
#[argh(subcommand)]
enum Subcommand {
One(SubcommandOption1),
Two(SubcommandOption2),
}

#[derive(FromArgs)]
#[argh(subcommand, name = "one")]
/// First subcommand
struct SubcommandOption1 {}

#[derive(FromArgs)]
#[argh(subcommand, name = "two")]
/// Second subcommand
struct SubcommandOption2 {}

fn main() {
let _args: Args = argh::from_env();
}
```

If the executable compiled from this code is run directly, without any args passed—`cargo run` or `./target/debug/text.exe`—the following help text is displayed:

```
One of the following subcommands must be present:
help
one
two

Run test.exe --help for more information.
```

This is somewhat inconsistent. On the one hand it supposedly needs one of those subcommands; on the other hand the recommendation-for-more-information is to run it *without* one of those subcommands, with only the `--help` flag.

Moreover, if one in fact follows either half of that advice—running it either with the `help` subcommand or the `--help` flag (which are, as far as I can tell, in practice synonymous)—then one will get *this* readout:

```
Usage: test.exe []

Example

Options:
--help display usage information

Commands:
one First subcommand
two Second subcommand
```

...wherein, suddenly, there's no `help` subcommand listed, while the `--help` flag is suddenly listed more centrally / less as-an-afterthought.

So, overall, it seems like there's some inconsistent behavior going on here. It doesn't seem like argh is quite sure whether it wants its help-menu-access to be primarily via subcommand (with the flag as a nondefault option for those who particularly need it) or whether it wants its help-menu-access to be primarily via flag (with the subcommand as a nondefault option for those who particularly need it). And I suspect that it would be better if argh were more committed in one direction or the other, rather than split halfway between the two as it currently is.

(All example code here was compiled and run on Windows 10, using argh 0.1.12.)