Print file name when searching for broken links

Summary:
Wasn't sure how to make it a one line wonder, so probably could be improved -- but slapped it into a while loops and print name before triggering the `markdown-link-check`.

Refers to #344 (comment)

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes #345

Differential Revision: D4352870

Pulled By: jessesquires

fbshipit-source-id: df62c7c29c7236cba5a9163819c56247dd2d5070

.travis.yml

@@ -50,7 +50,10 @@ script:
 
 - if [ $CHECK_MARKDOWN == "YES" ]; then
       npm install -g markdown-link-check;
-      find . -name "*.md" -exec cat {} \; | markdown-link-check;
+      find . -name "*.md" | while read filename; do
+        echo "Searching $filename";
+        cat "$filename" | markdown-link-check;
+      done
   fi