Add variables for graphql querues

[pacwoodson]

https://github.com/3box/3box-js/blob/a914ee905df1b0256ad278e0187a3f6eda03ca26/src/api.js#L221

Could you please add the possibility to pass variables to graphql queries ?

https://github.com/prisma-labs/graphql-request#using-variables

thanks