Outline command should be idempotent

[andreoliwa]

Problem

As a user converting old Markdown I want to run the outline command many times on the same file and always get the same output, so I don't need to care if the file is old or new, converted to outline or not.

Trying to convert to outline when the Markdown is already an outline should return the same results.

Possible solution

1. Make the outline command idempotent.
2. Detect if the line is already an outline and don't indent it again.