Add <command> section

[felixSchl]

Similar to [option], add a special section for commands:

Usage: foo <command>

Available commands:
  bar  lorem ipsum
          foo
  qux lorem ipsum

This extends docopt significantly.

This is a bit tricky to parse, but I think the most intuitive would be to mark the indentation level of the first
word after the actual command name. Then only treat text as part of the description of the command currently being parsed if it equal or more indented than the mark.