Support highlighting of visual indent guide (#135)

Previously only the `INDENT_GUIDE` editor scheme key was supported and
defined, but the `VISUAL_INDENT_GUIDE` key not that was added to the
IDE platform code in
JetBrains/intellij-community@2a17866.
Since both UI elements are almost the same, the new key now also uses
`nord3` as foreground color.

Closes GH-124

resources/themes/nord.xml

@@ -69,6 +69,7 @@
     <option name="VCS_ANNOTATIONS_COLOR_3" value="3b4252" />
     <option name="VCS_ANNOTATIONS_COLOR_4" value="3b4252" />
     <option name="VCS_ANNOTATIONS_COLOR_5" value="3b4252" />
+    <option name="VISUAL_INDENT_GUIDE" value="4c566a" />
     <option name="WHITESPACES" value="4c566a" />
     <option name="WHITESPACES_MODIFIED_LINES_COLOR" value="ebcb8b" />
   </colors>