Add option to load config from file

[Kamsiy]

Description
This PR allows loading a custom style from a json or yaml file or an environment variable

Examples

Using a loaded style with a Printer

style, err := printer.WithPrettyStyleFile("version-style.yml")
p := printer.New(style)

// or 
// to load from an environment variable
style, err := printer.WithPrettyStyleFromEnv("version-style")
p := printer.New(style)

Using a loaded style with Cobra

style := printer.WithPrettyStyleFile("version-style.yml")
cmd := &cobra.Command{
	Use:   "example",
	Short: "An example CLI built with github.com/spf13/cobra",
}
cmd.AddCommand(
	extension.NewVersionCobraCmd(
		extension.WithPrinterOptions(&style),
	),
)

Related issue(s)

• Add option to load config from file #13

[mszostok]

Thanks again for the PR, Awesome work! 🚀

The only reason that I didn't approve it is that it will cause a panic in such scenario:

func main() {
	// Add support for setting styles (optionally) via 'CLI_OUTPUT_STYLE' env
	styleFromEnv, err := printer.WithPrettyStyleFromEnv("CLI_OUTPUT_STYLE")
	if err != nil {
		log.Fatal()
	}

	p := printer.New(&styleFromEnv)
	if err := p.Print(os.Stdout); err != nil {
		log.Fatal(err)
	}
}

run:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xc0 pc=0x12ced80]

goroutine 1 [running]:
go.szostok.io/version/style.(*GoTemplateRender).Render(0xc0000122b8, {0x13e7c80, 0xc0001d40a0}, 0x98?)
        /Users/mszostok/workspace/go/src/github.com/mszostok/version/style/render.go:51 +0x940
go.szostok.io/version/printer.(*Pretty).execute(0x14e9a60?, 0xc000014018?, 0x83?)
        /Users/mszostok/workspace/go/src/github.com/mszostok/version/printer/pretty.go:89 +0x45
go.szostok.io/version/printer.(*Pretty).Print(0xc0000122d0, 0xc0001d40a0, {0x14e9a60, 0xc000014018})
        /Users/mszostok/workspace/go/src/github.com/mszostok/version/printer/pretty.go:71 +0x5f
go.szostok.io/version/printer.(*Container).PrintInfo(0xc00010fc80, {0x14e9a60, 0xc000014018}, 0xc0001d40a0)
        /Users/mszostok/workspace/go/src/github.com/mszostok/version/printer/printer.go:75 +0x70
go.szostok.io/version/printer.(*Container).Print(0xc000137f60?, {0x14e9a60, 0xc000014018})
        /Users/mszostok/workspace/go/src/github.com/mszostok/version/printer/printer.go:65 +0x3f
main.main()
        /Users/mszostok/workspace/go/src/github.com/mszostok/version/examples/custom-layout/main.go:18 +0xb2

When the env is specified, we cannot require that someone will always set it. It's up to the users to use it or not.

[Kamsiy]

Thanks, Good catch 😅. I've updated the PR to handle this

[mszostok]

LGTM 🚀 Thanks a lot for working on this 🙂