generate data file with list of apps with specific Anti-Features

When this plugin is parsing the index, this builds up a complete list of all apps that have a given Anti-Feature. This can be used to generate pages about apps with Anti-Features. Since the jekyll-polyglot model is to run a complete build of the site for each language, the only way I could think of to send the data was to write it out to a data file. This exposes the crazy pattern in jekyll-fdroid that it doesn't catch the parsed index, FDroid::IndexV1.download will parse it each time, so that means twice per language (for FDroidPackageDetailGenerator.rb and FDroidRepoInfoTag.rb). So what I've done here probably isn't pretty, but it works.

@obfusk can you check my naive Ruby? I hope I'm not doing something totally stupid.