Add description to ArgParser

[konsumlamm]

It would be nice to be able to provide a description for the ArgParser, that gets printed above or below the Usage: ... line in the help message.

This shouldn't be hard to implement, so I could make a PR if you like this idea.

[LepilkinaElena]

There was a plan to add DSL to allow users to construct help message if default one isn't suitable.

[konsumlamm]

There was a plan to add DSL to allow users to construct help message if default one isn't suitable.

Is this still planned?

[LepilkinaElena]

Yes, it's planned, but design isn't fully discussed.
cc @ilya-g