Make it possible to use the Snowman template and query engine from Go projects

[Abbe98]

While Snowman is a static site generator its RDF/SPARQL core is a query and a template engine. At the same time many developers who interested in Snowman operate projects at a scale in which it might be unwanted to relay entirely on static assets. We should therefore expose the template and query engines as public Go modules so that one can reuse these in traditional Go based backends.

An alternative would be to split the template engine and query engine from Snowman project entirely and leave Snowman to use them as dependencies.