Add RSS/Atom

[ArmorDarks]

There is no ready solution for Grunt yet. So, we should come up with our own.

Properties for xml could be gathered from Gray Matter or with cheerio.

Some useful resources:

• https://github.com/byteclubfr/gulp-rss
• https://github.com/jpmonette/feed
• https://github.com/dylang/node-rss

We also should remember about i18n...