Typesafe Config contribution of new feature .. "required includes"

Phew.

It's taken ages but I've finally managed to finish my contribution of a long standing functional enhancement to Typesafe config.

The new feature allows one to specify whether included resources are optional or mandatory. By default when Typesafe Config encounters a resource that it can't find then it carries on silently; this is by design.

However, often we want Typesafe Config to abort if certain includes are not found, and that's what I've added by way of a new keyword `required`.

The details of the change are here https://github.com/typesafehub/config/pull/421

The syntax for this is ...

include required("foo.conf")
include required(file("foo.conf"))
include required(classpath("foo.conf"))
include required(url("http://localhost/foo.conf"))