Support for snake_case

[CLAassistant]

All committers have signed the CLA.

[jcurr]

@mtsokol I would like to use your changes, would you be willing to sign the agreement so that the branch can be merged? Thanks

[mtsokol]

@jcurr sure, I've just signed it.

[rucas]

bump. would like to use this feature.

[henricook]

@mtsokol Next time it'd be really useful to update the README - especially for a breaking change like this.

To retain old behaviour should we be using implicit val encoder: NameEncoder = BaseNameEncoder()? Like this:

implicit val format: Format[MyCaseClass] = {
    implicit val encoder: NameEncoder = BaseNameEncoder()
    Jsonx.formatCaseClass[MyCaseClass]
  }