ReqTools tagging system

Now that should be clear how tagging system works, let's have a loot to ReqTools tagging system.

ReqTools tagging system offers two different kind of tags: those related to the whole ReqTools library and common to all ReqTools requesters, and those related to a specific requester and that have no effect if provided in a different one.

The common tags are discussed in detail in the common tags section, while the others are discussed while describing each the built in requester.