allow target[]=foo specification

[Dieterbe]

Rails/PHP/jQuery common practice format: target[]=path.1&target[]=path.2
Not really correct, but graphite allows it. So now we do also.

[Dieterbe]

interestingly, macaron allows for non-urlencoded [] also, so all 3 work:

GET /render?target=metrictank.stats.docker-env.default.memory.gc.last_duration.gauge64&from=-30s 
GET /render?target%5B%5D=metrictank.stats.docker-env.default.memory.gc.last_duration.gauge64&from=-30s
GET /render?target[]=metrictank.stats.docker-env.default.memory.gc.last_duration.gauge64&from=-30s

i also tested with multiple, though note the different ways of specifying cannot be combined. While we could trivially also allow that, I don't think we should be any more permissive wrt illegal syntax than graphite already is.