Extend the environment

[mattesmohr]

This pull request adds statements to the environment allowing views to be shown based on conditional terms or to loop over a sequence within an environment context, while it is unfortunately not feasible with native statements.

Environment.check(user.loggedIn) {
   Paragraph {
      "True"
   }
}

Environment.check(user.lastName == "Polo" && user.firstName == "Marco") {
   Paragraph {
      "True"
   }
} then: {
   Paragraph {
      "False"
   }
}

Environment.loop(user.roles) { role in
   Paragraph {
      role
   }
}

Environment.unwrap(user.avatar) { avatar in
   Image()
      .source(avatar)
} then: {
   Paragraph {
      "No image found"
   }
}

I also want to state that I am not completely satisfied with the naming yet and may change it in the future.