.myntignore feature

Hi!

I faced some problems trying to build a site on GitHub Pages: site have to be built to repository root, but ``mynt gen`` command completely empties build destination dir, which may actually contain some extra files like ``README``, ``Makefile``, etc. So I used temporary ``.build`` dir and then rsync-ed it with my root.

That seems I've found a better solution - placing ``.myntignore`` file with list of protected files & dirs to destination dir. Now I don't need temporary dir and can build directly to my repo root.

This also allows me to build another mynt project to sub-directory - I just need to add this sub-directory to ``.myntignore`` so main project gen-script won't remove it. Use case: if I'd wanted to get different blogs for different languages.

What would you say? :)