Add an option to control max indentation depth

[olivier-lacroix]

Hello there!

Thanks for this useful crate!

This PR adds an option to control the maximum indentation level for subcommands:

• The first level of subcommands are not indented (ie. they have the same level as the main command)
• The max_depth option control how deep subcommands can be indented

Also a bit of drive-by cleanup:

• removed unecessary returns
• deleted commented code
• made help_markdown DRYer