Cobalt processes vim .swp files in _layouts as templates, and warns

[skierpage]

(This is similar to issue #556)

If you use vim to edit a file in _layouts, Cobalt processes vim's binary .<FILE>.swp swap file(s) as if they were liquid templates, and cobalt serve complains about their content:

Building from `/tmp/myBlog` into `/tmp/.tmpxwRaPV`
WARN: Failed to load layout .default.liquid.swp

As @lazysoundsystem commented on 556, I think it happens because fn find_layouts() sets ignore_hidden(false) (I haven't tried their one-line fix yet). Processing files is different from serving pages, but it seems unnecessary and counterintuitive that Cobalt supports templates files starting with . or _ in _layouts.