Allow custom ordering of args and subcommands in help message

[kbknapp]

I'm imagining adding something like a display_order (or whatever it gets called) method which sets the display order. Items with the same order will be listed alphabetically and the default will be something like 999 so the custom orders appear first and have a low probability for being accidentally mixed in with defaults. This would also allow some overlap so that maintenance should be low.

This should help when there are arguments or subcommands you'd like to highlight or that are most frequently used and you don't want to overwrite the entire help message by hand.

Such as something like is discussed in multirust-rs

[kbknapp]

Implemented by #443