pick YASR plugin using query comment

[VladimirAlexiev]

I understand that CONSTRUCT/DESCRIBE are handled by the YASR Graph plugin, due to priority. But a hypothetical "tree" viz plugin may also serve CONSTRUCT. And how can a query request eg Geo?

The Wikidata query service has a more flexible idea: it can select which visualization to use through a special comment on the first query line.

This would be useful for #133

Note: prefix auto-addition should do it after initial comment lines, not before.

[MathiasVDA]

Having some way to have a more intelligent default plugin selection for a query is a good idea. I'm not sure about the use of a comment for this but right now I also don't have a better idea.