Don't tweak src URL of img links, if absolute

[jennybc]

vroom sources taylor.gif from a raw GitHub link in README / index.html. The gif used to live in gh-pages, but that's not compatible with clean = TRUE in deploy_to_branch(), i.e. it keeps going missing.

I tried relocating the gif to man/figures and vignettes, but the link remained broken in the rendered site. I then realized that the link was being tweaked by pkgdown to a nonexistent path, when transforming README.md into index.html.

Based on the comment above tweak_img_src() (# Fix relative image links), I'm pretty sure this PR is compatible with the original intent.

[juniperlsimonis]

thank you so much for tracking this down, @jennybc
just caught this problem in some packages' pkgdown pages and had not been able to figure out what the heck is up

[hadley]

Would you mind adding a news bullet please?

[hadley]

Thanks Jenny!