Support "description" tag

[ogerardin]

Help usage generation would be improved if struct members supported a "description" tag (or a sub-tag of "config"), e.g.

type Configuration struct {
	Port          int    `config:"port" description:"listening port number"`
}

or

type Configuration struct {
	Port          int    `config:"port,description=listening port number"`
}

[steamrolla]

The above pull request should be closed as the flags backend has changed since it's submission.

A new pull request has been submitted.