fix(UsageText): consistent indent for help UsageText output

[clok]

What type of PR is this?

• bug
• cleanup
• documentation
• feature

What this PR does / why we need it:

This PR addresses the bug found by @codefromthecrypt and noted in #1278

Which issue(s) this PR fixes:

Fixes #1278

Testing

• Added multiple unit tests that test the functionality for multi-line UsageText values for App, Command and SubCommand types.
• Spot tested with a local CLI tool integration.

Release Notes

fix(UsageText): consistent indent for help UsageText output

[codefromthecrypt]

lovely!

[codefromthecrypt]

only thought is might want to also backfill a test that MarkdownDocTemplate does not indent any of the lines (maybe overkill..)

[clok]

only thought is might want to also backfill a test that MarkdownDocTemplate does not indent any of the lines (maybe overkill..)

The markdown is a bit different. The values passed into the template are manipulated before the execution of the template render. Note the prepareCommands on line 59 as an example. https://github.com/urfave/cli/blob/master/docs.go#L56-L62

As a result, there shouldn't be a need to adjust the template or test for the MD generation. A part of me wants to refactor the MD generation to use template functions more consistently, but that is an out of scope effort that I think warrants more conversation before implementing.

[codefromthecrypt]

out of scope comment so hope it isn't too distracting. I found indeed markdown could be better handled differently, though to each their own on this. Particularly, there's formatting bias towards help output which doesn't work so well in markdown. I ended up using this which means I don't personally need the markdown feature at the moment:

	cli.MarkdownDocTemplate = `# GetEnvoy CLI Overview
{{ .App.UsageText }}

# Commands

| Name | Usage |
| ---- | ----- |
{{range $index, $cmd := .App.VisibleCommands}}{{if $index}}
{{end}}| {{$cmd.Name}} | {{$cmd.Usage}} |{{end}}
| --version, -v | Print the version of GetEnvoy |

# Environment Variables

| Name | Usage | Default |
| ---- | ----- | ------- |
{{range $index, $option := .App.VisibleFlags}}{{if $index}}
{{end}}| {{index $option.EnvVars 0}} | {{$option.Usage}} | {{$option.DefaultText}} |{{end}}
`

[codefromthecrypt]

fixes the issue and markdown is out-of-scope!