Add topics concept

[brunoarueira]

Different from #5 , which will allow each reader set their own category about a feed registered, the topic will be suggested through a method described at #9 (about scrap, read metadata or artificial intelligence).

Example:

• Blog about software development with more technical stuff, such as implementations or the way to do things, will detect the techs described and apply topics.

In the future, the topics probably will be suggested to help the users define their categories as described #5, but for now the topics will be a meta aggregator.