added arg_name attribute, repeatable options are now documented

[mkatychev]

repeated #[argh(option] arguments are now shown in the help header appropriately:

#[argh(option)]
/// option
opt: Vec<String>,

• old output: [--opt <opt>]
• new output: [--opt <opt...>]

Option names can now be overridden with the arg_name = "string" attribute:

#[argh(option, arg_name = "opt_arg")]
/// option
opt: String,

• produces: --opt <opt_arg>

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[mkatychev]

@googlebot I signed it!