Find variables in _config.yaml by searching with relative file path.

This basically will fix https://github.com/jekyll/jekyll/issues/2371. Please check/reproduce there.

When FrontmatterDefaults tries to find matching sets from config file, lookup by path fails if it also contains "_posts" part. So instead of matching path like "/all/pages/2014-01-01-test.md" it will try to also match "/all/pages/_posts/2014-01-01-test.md".

Without the fix, we won't be able to customize via _config.yml files not assigned to any category (located at root /_posts)