refactor rendering backends

[pietroppeter]

this is a breaking change that is currently the main target for 0.4.

implementing a new backend is currently very messy and a new approach needs to be used that it will allow to:

• allow multiple backends to be run at runtime (currently the only two backends - html and md - need a compile time switch to be used both)
• improve markdown backend
• use a different html backend e.g. based on nimja
• implement a latex backend (or a pdf backend based on that)
• implement a json backend
• implement a jupyternim backend
• ...

related:

• replace context: mustache.Context object with data: JsonNode object #102