Add json-ld templating for feature collection item

[webb-ben]

Addressing https://github.com/cgs-earth/pygeoapi/issues/1, this PR adds the capacity to render complicated json-ld templates using jinja2 and the default json-ld output from pygeoapi.

• Check for template when generating JSON-LD document for an individual collection item.
• Default @id instead of aliasing with @context. Makes easier to render jinja2 template with the appropriate fields.
• Rename default geosparql to json-ld convention gsp
• Include basic json-ld template generation in test workflow.
• Add documentation for templating.

[tomkralidis]

Great work @webb-ben! See change requests. I've also added @alpha-beta-soup to review.