$validate_links() method: differentiate between images and links in report summary

[zkamvar]

This will differentiate between images and links in the report summary. In the case where only images are failing a11y checks, the user will see

! There were errors in 3/80 images
◌ Images need alt-text <https://webaim.org/techniques/hypertext/link_text#alt_link>

If there are only errors in the links and not any images, then the user will see something like this

! There were errors in 1/80 links
◌ Links must use HTTPS <https://https.cio.gov/everything/>

If there are both, then the user will see something like this

! There were errors in 4/80 links and images
◌ Images need alt-text <https://webaim.org/techniques/hypertext/link_text#alt_link>
◌ Links must use HTTPS <https://https.cio.gov/everything/>

This will fix #143