Support variable categories

[stevenroose]

I was thinking it would be nice to have support for categories. Especially for the --help output.

The problem, however, is that pflag does not support them right now and we rely on them to generate the usage string: https://github.com/spf13/pflag/blob/master/flag.go#L649

I thought about refactoring that function in pflag to an auxiliary one that returns the individual lines in a []string and then grouping and formatting ourselves from there.

[stevenroose]

e609545 implements our own help printing function.

I'm wondering how variable categories could be implemented.

First of all, a HelpCategoriesForStructs would allow a 1-line header for a set of flags that is nested in a struct. Like this:

var config = struct {
	IntVar int `id:"intvar" desc:"some random integer" default:"42"`
	DbVars struct {
		Name string `id:"name" desc:"database name"`
		User string `id:"user" desc:"database user"`
	} `id:"db" desc:"Database variables"`
}{}

The current output

Usage of test:
      --intvar int       some random integer (default 42)
      --db.name string   database name
      --db.user string   database user
  -h, --help             print this help menu

Could become then something like this:

Usage of test:
      --intvar int       some random integer (default 42)

Database variables:
      --db.name string   database name
      --db.user string   database user

  -h, --help             print this help menu

A second approach could be to allow more control f.e. with a cat:"db" in the tag and then a HelpCategories map[string]string option in the config.