Add configuration allow_multiple_hashes to CommentFormat

[axelson]

(this is a reimplementation of #222)

Currently, Dogma raises a CommentFormat error for comments that have multiple '#' characters before the beginning of the comment eg ## Currently invalid comment.

This PR adds an allow_multiple_hashes configuration to the CommentFormat rule that allows multiple # at the beginning of a comment.

Points of reference in favour of allowing multiple # characters before a comment:

• When generating a new Phoenix app, in the web/channels/user_socket.ex file, for example, you'll see comments starting with ## generated from its template
• Rubocop's leading comment space cop allows for multiple # characters

[axelson]

@lpil Is there anything needed to improve this PR?