* Implement Virus property
It allows the developer to make custom transformations to the genotype
before being parsed by Cell

* (wip) Fail when a mutation does not adhere to the API

* Add  feature as a plugin mechanism

* Add type by default after virus infection
This fixes the problem that the node was generated with a type
but the virus tried to override it to a div implicitly

* Fix missing semicolon

* Move virus infection up in the pipeline

Infecting in Genotype.build may be too late, as the $node is already
generated. This caused issues when writing viruses that changed the $type
of cells initially declared with "$type: 'input'".
This is also cleaner, as viruses act like dumb pre-processors or macro
expansions.

* Add Virus documentation

* Update VIRUS.md