cli: public commands API

[anpep]

Introduce a public API for well-known Pebble-based applications to use.
This PR introduces two basic changes on top of those included in #226:

• Export the cli.AddCommand() function, so that applications can add new commands.
• Export the cli.HelpCategories variable, so that applications can add new sections to the help manual.

// Example: adding a help category
package main

import "github.com/canonical/pebble/internals/cli"

var myCategory = cli.HelpCategory {
    Label: "My category",
    Description: "Description for my help category",
    Commands: []string{"new-command"}
}

func main() {
    cli.HelpCategories = append(cli.HelpCategories, myCategory)
    cli.Run()
}

// Example: adding a new command
package main

import (
    "fmt"

    "github.com/canonical/go-flags"
    "github.com/canonical/pebble/internals/cli"
)

type cmdNewCommand struct {
    Force bool `short:"f" long:"force"`
}

func init() {
    cli.AddCommand(&cli.CmdInfo{
        Name:        "new-command",
        Summary:     "Lorem ipsum. Foo bar.",
        Description: "Dolor sit amet.",
        Builder:     func() flags.Commander { return &cmdNewCommand{} },
        ArgsHelp: map[string]string{
            "--force": "Forces the operation",
        },
    })
}

func (cmd *cmdNewCommand) Execute(args []string) error {
    if len(args) > 0 {
        return cli.ErrExtraArgs
    }
    fmt.Fprintf(cli.Stdout, "Hello, world!\n")
    return nil
}

[atesburak]

Minimal change to make AddCommand public, looks good to me with an extremely minor comment.

[paul-rodriguez]

Logical follow-up step on #226, looks good to me.

[flotter]

This is the last change required on the Pebble side before we can create a different variant of Pebble in a separate repository.

Some work remains to allow for personality changes to update string references to "pebble" in the help, but that will be a followup PR.

[niemeyer]

Looks mostly okay since it's a private use, but I still hope we can improve it slightly as outlined below. Bonus points if there's a test for it.

[niemeyer]

Thanks for moving it forward. There are still remaining issues related to the points made in the prior review it seems.

[flotter]

Some subjective preferences, but otherwise looks great!

[niemeyer]

Here are a few extra details, but it looks like the points discussed last time about debug commands still haven't been touched?

[niemeyer]

Thanks Angel. One more pass.

[niemeyer]

Almost there!

[niemeyer]

Thanks for all the work, Angel. Hopefully this is ready to go in now.