Allows custom ordering of args within the help message. Args with a lower value will be
displayed first in the help message. This is helpful when one would like to emphasise
frequently used args, or prioritize those towards the top of the list. Duplicate values
**are** allowed. Args with duplicate display orders will be displayed in alphabetical
order.

**NOTE:** The default is 999 for all arguments.

**NOTE:** This setting is ignored for positional arguments which are always displayed in
index order.

```rust
use clap::{App, Arg};
let m = App::new("cust-ord")
    .arg(Arg::with_name("a") // typically args are grouped by alphabetically by name
                             // Args without a display_order have a value of 999 and are
                             // displayed alphabetically with all other 999 args
        .long("long-option")
        .short("o")
        .takes_value(true)
        .help("Some help and text"))
    .arg(Arg::with_name("b")
        .long("other-option")
        .short("O")
        .takes_value(true)
        .display_order(1)   // Let's force this arg to appear *first*
                            // all we have to do is give it a value lower than 999
                            // any other args with a value of 1 would be displayed
                            // alphabetically with other 1 args. Then 2, then 3, etc.
        .help("I should be first!"))
    .get_matches_from(vec![
        "cust-ord", "--help"
    ]);
```

The above example displays the following help message

```
cust-ord

USAGE:
    cust-ord [FLAGS] [OPTIONS]

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
    -O, --other-option <b>    I should be first!
    -o, --long-option <a>     Some help and text
```

Allows custom ordering of subcommands within the help message. Subcommands with a lower
value will be displayed first in the help message. This is helpful when one would like to
emphasise frequently used subcommands, or prioritize those towards the top of the list.
Duplicate values **are** allowed. Subcommands with duplicate display orders will be
displayed in alphabetical order.

**NOTE:** The default is 999 for all subcommands.

```rust
use clap::{App, SubCommand};
let m = App::new("cust-ord")
    .subcommand(SubCommand::with_name("alpha") // typically subcommands are grouped
                                               // alphabetically by name. Subcommands
                                               // without a display_order have a value of
                                               // 999 and are displayed alphabetically with
                                               // all other 999 subcommands
        .about("Some help and text"))
    .subcommand(SubCommand::with_name("beta")
        .display_order(1)   // In order to force this subcommand to appear *first*
                            // all we have to do is give it a value lower than 999.
                            // Any other subcommands with a value of 1 will be displayed
                            // alphabetically with this one...then 2 values, then 3, etc.
        .about("I should be first!"))
    .get_matches_from(vec![
        "cust-ord", "--help"
    ]);
```

The above example displays the following help message

```
cust-ord

USAGE:
    cust-ord [FLAGS] [OPTIONS]

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

SUBCOMMANDS:
    beta    I should be first!
    alpha   Some help and text
```

Closes #442

Closes #439