bin/lesson_check.py: allow exceptions to line length limit

[maxim-belkin]

Allow lines that contain a single image or a single link to go over the suggested line length limit. So, all of the following lines will be allowed to go over the line limit:

[alt text](/some/url)
![alt text](/path/to/image)
[alt text][internal_link]
![alt text][internal_link_to_image]

Plus all of the above prefixed with any combination of > and to allow these lines to go over the limit in various code blocks.

Fixes #591

[fmichonneau]

Thanks Maxim!

[maxim-belkin]

Thanks, François!

@jhlegarreta, could you please confirm that after this change make lesson-check-all doesn't complain about the long lines that contain images or links only?

[jhlegarreta]

The regex does not seem straightforward ! Thanks both 👍.

@jhlegarreta, could you please confirm that after this change make lesson-check-all doesn't complain about the long lines that contain images or links only?

Will give it a try when I find the time (will try during the weekend) and will report back.

[jhlegarreta]

Unfortunately, it does not look like it solved the issue. In fact, the number of warnings has increased, e.g.:
https://github.com/carpentries-incubator/SDC-BIDS-dMRI/runs/2531577900?check_suite_focus=true#step:16:20
with respect to an immediately previous version, e.g.
https://github.com/carpentries-incubator/SDC-BIDS-dMRI/runs/2528767166?check_suite_focus=true#step:16:20

In the previous version, link-only lines (e.g. line 190 at the pointed file - __episodes/constrained_spherical_deconvolution.md) were not triggering warnings, and now they are. Meanwhile, the lines that made me open (e.g. line 458) the issue are still raising the warnings.

[maxim-belkin]

Oh, I see what the problem is. The pattern I added doesn't allow {:class ...}at the end. I'll fix that.

[maxim-belkin]

So, I found a few things that you use in your lesson that I didn't take into account when I was working on this pattern:

1. You use links in headings
2. You use {:class=...} customizations in link/image-only lines
3. You use \ at the end of the image/link-only lines

So, #597 should address all of these.