Implement docs generator

[antonmedv]

This feature will be part of Deployer v7.

What should be done:

• dep command dep docs [recipe-file-path.php]
• output: Markdown

Idea:

• Parse recipe and generate markdown docs.
• The command takes a path to the recipe, parses PHP for comments and generates markdown.
• What should be in generated markdown:
  • First comment of a file (aka description of recipe). Starting with /** or /* or //.
  • Each task('...') -> collect desc() and comment right before task.
  • Each set('..') -> as list of options as well as comment right before set.
  • Links to other required recipes.

[antonmedv]

I did it myself.
47a193f