Handle jsonrpc context

[fiddur]

Context specification needs to be better before implementing this correctly.

We have an untested implementation commented out in 1dc552c

The idea is to be able to make a context something like this:

"context": {
  "webshopApi": {
    "type": "prerequisite",
    "description": "API backend URL for jsonrpc"
  },
  "article": {
    "type": "jsonrpc",
    "endpoint": "{{context.webshopApi}}",
    "method": "Article.get",
    "params": ["{{query.article}}", ["name", "url", "description"]]
  }
}

…this would require query (GET-data) to be placed on mustache rendering context as well.