Add missed 'html' to 'format' option explanation and arrange formatters in alphabetical order

[strangeman]

Before:

inspec help exec
...
      [--format=FORMAT]                            # Which formatter to use: cli, progress, documentation, json, json-min, junit

After:

inspec help exec
...
      [--format=FORMAT]                            # Which formatter to use: cli, documentation, html, json, json-min, junit, progress

I found that inspec can produce reports in HTML format from StackOverflow thread, there is no info about it in official documentation. I think we should fix it.

[adamleff]

Hi, @strangeman! Thanks for your contribution to InSpec!

HTML output is currently provided "automatically" because RSpec provides it... it's not actually an InSpec feature itself. We may not support it as-is in the future, but for now, your change makes sense and there's no reason to hide that formatter.

Thank you for your PR!