Make "a-z" characters work in filter hints mode.

When we read hint characters, we read them lower case.  When we generate
hint markers, we generate them upper case.  So they never match.

(Exactly why anyone would want to use "abcde" for filtered link hints
isn't clear, but at least we should behave correctly.)

content_scripts/link_hints.coffee

@@ -532,7 +532,7 @@ class FilterHints
     # input. use them to filter the link hints accordingly.
     matchString = @hintKeystrokeQueue.join ""
     linksMatched = @filterLinkHints hintMarkers
-    linksMatched = linksMatched.filter (linkMarker) -> linkMarker.hintString.startsWith matchString
+    linksMatched = linksMatched.filter (linkMarker) -> linkMarker.hintString.toLowerCase().startsWith matchString
 
     if linksMatched.length == 1 && @hintKeystrokeQueue.length == 0 and 0 < @linkTextKeystrokeQueue.length
       # In filter mode, people tend to type out words past the point needed for a unique match. Hence we