Add more keywords (#12)

This PR adds extra keywords for the Kotlin language, hopefully this is
correct. Otherwise please point me to the right place.

Author: martinbeentjes

languages/kotlin/highlights.scm

@@ -279,6 +279,9 @@
   "class"
   "object"
   "interface"
+  "companion"
+  "package"
+  "import"
 ; "typeof" ; NOTE: It is reserved for future use
 ] @keyword