Insertion concept and splitting blocks and tags logically.

[msangel]

1. Created basic class Insertion as a common parent for both Block and Tag.
2. Block no longer extends Tag.
3. All blocks moved to package block.
4. Most of the logic internally with blocks and tags is performed via Insertion abstraction.
5. Custom blocks/tags/filters registration is made in ParseSettings object. Method .with(tag/block/filter) reverted and moved there.
6. Documentation updated to reflect the changes.

@bkiers @sdlin @honglooker

[honglooker]

Thanks for the cleanup!