Only validate links

[hoijui]

With a command line switch, like --only-validate-links, only that last phase of the process would be done, and the list of links would have to be provided, instead of an input directory.

This can be useful for debugging, for using other tools for doing the link-extraction step (e.g. pandoc or the like, which support more formats), or .. who knows what!
It would move mlc closer to the UNIX style of software, and give more power to the users of this software.