Initial commit

As is, this doesn't work, because there is a disagreement
between the declared API of /validations and what is
actually implemented in the apidoc server.
How about that validate() takes a File as a parameter, which is given by `apidocSource`, a settingKey[File].
Its default value might be `resourceDirectory.value / "api.json"`

Also, I think `sbt.IO` tends to be used within SBT code, e.g. `sbt.IO.read`