Add a "context" var resolving to the verticle context

[vietj]

Currently it's not possible to obtain directly the verticle context in JavaScript like the vertx object

[DanielJoyce]

Currently works fine in 3.3.2

var cfg = Vertx.currentContext().config();

A note to others looking trying to grab configuration passed via -conf

[StephenOTT]

@DanielJoyce is there a reason why this conflicts the examples provided in the JS docs ? (vert-x3/vertx-config#39)