It should handle directories by default

[gaurav-nelson]

At present, it requires us to manually run it on directories using the command
find . -name \*.md -exec markdown-link-check {} \;

I believe this should be handled automatically. So that when user runs it on a directory, it should check all markdown files recursively by default.

$ markdown-link-check -p run/on/dir

Let me know if this is required? I can put together up a PR to address this.

[nschonni]

Duplicate of #40 I think

[NicolasMassart]

duplicates #40