add support for dashed arg names and values in arg! macro

[evgeniy-terekhin]

While migrating from clap2 to clap3 I ran into lack of support for dashed arg names and values in clap::arg! macro
meanwhile such definitions were supported but functions like arg_from_usage

Here are some examples of such problematic argument usage descriptions

[num-readers] --num-readers=<n> 'Number of reader threads'
[num-loaders] --num-loaders=<n> 'Number of loader threads'

(another problem I found is that arg! macro does not allow [flag] before any of -f or --flag but that is a different story)

so my proposed just repeats what is already done for longarg
makes it possible to wrap arg name and arg value in double quotes so that it is interpreted as literal rather than an ident

thus arg!([some_arg] "some arg") becomes possible this way arg!(["some-arg"] "some arg")
and arg!(<some_val> "some val") becomes possible this way arg!(<"some-val"> "some val")

P.S some tests provided