Merge: nitmetrics: metrics about ReadMe files

Merge: nitmetrics: metrics about ReadMe files

This PR add an option `--readme` to extract metrics about ReadMe / Markdown files.

Running the following command:

~~~sh
nitmetrics --readme tests/test_prog
~~~

Will produce the following output:

~~~md
*** METRICS ***

# ReadMe metrics

 ## package test_prog (test_prog/README.md)
  * has_package 1
  * has_readme 1
  * md_lines 12
  * nb_section 0
  * md_blocks 14
  * BlockParagraph 4
  * BlockUnorderedList 1
  * BlockListItem 4
  * SpanCode 4
  * Image 1
~~~

Pull-Request: #2634
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>