Argument specification (in usage-line) customization & "smart" default

[z33ky]

I find the defaut $argv0 [OPTIONS] is very bare bones.
A custom spec can now be passed using a new arg_spec attribute:

#[derive(Options)]
#[options(arg_spec = "MY [SPEC]")]
struct MyOptions { ... }

Which shows up in the first usage line:

$ target/debug/test
Usage: target/debug/test MY [SPEC]
...

I've also added some smart option detection, so the default spec can now show COMMAND, OPTIONS and ARGS, with a ... if multiple are valid and surrounded by [] if optional.

If you find this useful I will update the documentation and would also add tests before considering this ready to merge.

I would also like to allow more customization, like formatting the strings as command or <command> instead of fixating on the current UPPER_CASE.
My ultimate vision would also allow generation of a "full" command-spec like Usage: $argv0 [-h|--help] [-v] [--file FILE] [-- PATHS...]. This does get unwieldy when a tool has lots of options, so it should definitely be optional. Maybe a user can also tag "important" options to include in the usage-line and leave out the rest.
If I add these I will implement them in separate PRs.