Create front-end client lib to fetch graph data

[grtjn]

Create a front-end library that uses fetch to call mid/back to fetch graph data conform specs that will be spelled out in #4. The library should be plain JS (e.g. useable in all ui frameworks).

It could also be a placeholder to do some extra leg work for post processing if the need for something like that would arise.

The library would not do any caching. If front-end caching would be needed, UI application state should be used for that.

[grtjn]

GraphApi.js could be used as inspiration perhaps, though the sparql probably belongs in middle-tier or backend..

https://github.com/marklogic-community/grove-vue-visjs-graph/blob/master/src/api/GraphApi.js