* Reduces some of the code duplication between the `encode` and `encodePretty` functions
* The output of `encodePretty` has been improved:
- Multiline strings now use `Literal` style instead of `SingleQuoted`
- Special keys are now quoted in mappings [#179](https://github.com/snoyberg/yaml/issues/179)
* Support for complex keys in mappings: [#182](https://github.com/snoyberg/yaml/issues/182)
- Adds `complexMapping` function to `Data.Yaml.Builder`
- Decode functions now return a `NonStringKey` error when attempting to decode a mapping with a complex key as it is not possible to decode these to an Aeson `Value`
