Refactor model generator to use Nunjucks

[nickevansuk]

Taking inspiration from this project:

• https://github.com/asyncapi/generator

Which uses template files like this:

• https://github.com/asyncapi/html-template

We should refactor to use https://mozilla.github.io/nunjucks/

This provides a clean separation between the generator which creates a data structure containing all the info relating to the properties, and the logic that generates the actual language specific files.

The idea is that any time additional data is needed in the data structure to accommodate a new language, this can be added to the generator. However the logic that generates the files is neatly packaged in each template.

This separation of concerns increases maintainability, and makes #29 easier