Support highlighting of selected indent guide (#134)

The selected indent guide was previously not highlighted differently
like non-selected guides (`nord3`).
This has been changed to use the same color like comments (`nord3` with
increased brightness) instead by adding support for the
`SELECTED_INDENT_GUIDE` editor scheme key added to the IDE platform code
in JetBrains/intellij-community@7e3a238.

Closes GH-123

resources/themes/nord.xml

@@ -49,13 +49,15 @@
     <option name="HTML_TAG_TREE_LEVEL5" value="" />
     <option name="INDENT_GUIDE" value="4c566a" />
     <option name="INFORMATION_HINT" value="434c5e" />
+    <option name="LINE_NUMBER_ON_CARET_ROW_COLOR" value="d8dee9" />
     <option name="LINE_NUMBERS_COLOR" value="4c566a" />
     <option name="METHOD_SEPARATORS_COLOR" value="d8dee9" />
     <option name="MODIFIED_LINES_COLOR" value="ebcb8b" />
     <option name="NOTIFICATION_BACKGROUND" value="4c566a" />
     <option name="QUESTION_HINT" value="434c5e" />
     <option name="RECURSIVE_CALL_ATTRIBUTES" value="" />
     <option name="RIGHT_MARGIN_COLOR" value="4c566a" />
+    <option name="SELECTED_INDENT_GUIDE" value="616e88" />
     <option name="SELECTED_TEARLINE_COLOR" value="434c5e" />
     <option name="SELECTION_BACKGROUND" value="434c5e" />
     <option name="SOFT_WRAP_SIGN_COLOR" value="4c566a" />
@@ -69,7 +71,6 @@
     <option name="VCS_ANNOTATIONS_COLOR_5" value="3b4252" />
     <option name="WHITESPACES" value="4c566a" />
     <option name="WHITESPACES_MODIFIED_LINES_COLOR" value="ebcb8b" />
-    <option name="LINE_NUMBER_ON_CARET_ROW_COLOR" value="d8dee9" />
   </colors>
   <attributes>
     <option name="ABSTRACT_CLASS_NAME_ATTRIBUTES">