Skip processing for files served with content-type of "text/html"

Github, Bitbucket, etc serve URLs with ".md" extension in their HTML-converted form.
E.g. https://github.com/Thiht/markdown-viewer/blob/master/README.md
Merge pull request #30 from ayemooth/skip-text-html-files

Skip processing for files served with content-type of "text/html"