Add support for FLYWAY_CONFIG_FILES option in build.sbt

[aleksandr-vin]

As I already have a config file, I want a one setting in build.sbt to configure flywayPlugin to source config from that file.

[aleksandr-vin]

The workaround could be:

flywayDefaults := flywayDefaults.value.configuration {
  import org.flywaydb.core.internal.configuration.ConfigUtils
  import java.io.File
  ConfigUtils.loadConfigurationFile(new File("src/main/resources/flyway/flyway.conf"), "UTF-8", true)
}