add aeson json parser.

[julialongtin]

CuraEngine configurations are stored in json. implement reading json, so we can use the settings from cura.

[jaredpmclaughlin]

The long form explanation is ... CuraEngine generates gcode for 3D printers ( from what? probably STL ). It has a configuration files in json, and it seems like folks have developed a bunch of them for different machines, so it makes sense just to adopt their file format for configuration values. Aeson seems to be a parser. I don't actually know where the values need to go.