Optimize code #25

I've noticed that unmarkers.BaseUnmarker been documented as an "abstract base class" when we're actually using it otherwise.

Also, there's some dead code and we should actually sprinkle @staticmethod on some of them.

Here's my idea:

• Move all the tag_* methods in BaseUnmarker ➡️ BasicUnmarker
• Rename: BaseUnmarker ➡️ AbstractUnmarker
• Alias: BaseUnmarker ➡️ BasicMarker
• Run shed on the whole codebase (with --refactor)

Version bump: minor