support aditional options for json renderer

[behrica]

Library should support enforcing json conversion like this (and other options):

curl https://public.opencpu.org/ocpu/tmp/x017649f34f/R/.val/json?force=true

[jeroen]

To see the currently supported arguments for json conversion: http://public.opencpu.org/ocpu/library/jsonlite/R/toJSON/print