Configure automatic source formatting

[jonas]

At some point we should adopt the configuration and scripts used by Scala Native for consistency:

• https://github.com/scala-native/scala-native/blob/master/scripts/clangfmt
• https://github.com/scala-native/scala-native/blob/master/.clang-format
• https://github.com/scala-native/scala-native/blob/master/scripts/scalafmt
• https://github.com/scala-native/scala-native/blob/master/.scalafmt.conf

And check via Travis CI.