Add `HostOS` utility

This currently calls out to a private JavaFX utility from Sun. If that
changes, I'll modify the code to use the system properties directly.

src/main/scala/org/gerweck/scalafx/util/HostOS.scala

@@ -0,0 +1,13 @@
+package org.gerweck.scalafx.util
+
+import com.sun.javafx.PlatformUtil
+
+/** Utilities for identifying the Host OS of the application.
+  *
+  * @author Sarah Gerweck <sarah.a180@gmail.com>
+  */
+object HostOS {
+  lazy val isMac = PlatformUtil.isMac()
+  lazy val isUnix = PlatformUtil.isUnix()
+  lazy val isWindows = PlatformUtil.isWindows()
+}