Add 'display_name' as new attribute for parameters

[krixkrix]

• this is intended to be shown in a user interface instead of the raw parameter name
• compatible with current config definitions as it defaults to an empty string

implements feature #110

connects to #110

[mikaelarguedas]

Thanks @krixkrix for the contribution!

Please see https://github.com/ros/dynamic_reconfigure/issues/110#issuecomment-422520138 for general comment about the approach.

The minimal dump seems unrelated to this change, would you mind opening a separate PR for it ?

[krixkrix]

The minimal dump has now its own pull request: #123