Only add loading=lazy if I didn't already specify it on the img.

voussoir.net/writing/vmarkdown.py

@@ -654,6 +654,7 @@ def inject_footnotes(soup):
 def set_img_lazyload(soup):
     imgs = soup.find_all('img')
     for img in imgs:
+        if not img.get('loading'):
             img['loading'] = 'lazy'
 
 # FINAL MARKDOWNS