Raise error for links with "Click Here" as text

Related with https://github.com/swcarpentry/lesson-example/issues/175.

This need to be implemented at https://github.com/swcarpentry/lesson-example/blob/gh-pages/bin/lesson_check.py. We should parse the Markdown file looking for

~~~
[click here]()
~~~

on it and raise a error if found.

Start in bin/lesson_check.py and inspect how Markdown files are currently parsed and reported. Add coverage for links whose visible text is "click here" and confirm that the checker raises an error for the example pattern shown in the issue.

Written by the indexing model from the issue text.