Implement "proof-of-concept" rst2rst command

[benoitbryon]

See original feature request at benoitbryon/documentation-style-guide-sphinx#8

Proof of concept implementation could be:

Feature: script which transforms RST files to RST
  In order to apply style guide conventions on existing RST files
  As a documentation writer
  I want to run a script which does most of the work automatically

  Scenario: apply style guide recommendations on an existing RST file
    Given a file named `example.txt`
    And this file contains valid RST content
    And the title of the document doesn't use `#` (sharp) character with overline
    And other titles don't use `#` (sharp) character
    When I run `rst2rst` script with `example.txt` as argument
    Then the RST content is displayed on the standard output
    And the title of the document uses  `#` (sharp) character with overline.

[benoitbryon]

Done. The implementation is really naive right now... but should be enough for a proof of concept.