Fix done.txt naming bugs

Two unreported done.txt naming bugs fixed:

1. *.Todo.txt files would use *.done.txt for the done filename

2. When a user had the ignorecase option set, the done.txt file was
   always named Done.txt, because the == operator ignores the case when
   this option is set.

The bugs were introduced in PR #73

autoload/todo/txt.vim

@@ -71,7 +71,7 @@ function! todo#txt#remove_completed()
     " Check for user-defined g:todo_done_filename
     if exists("g:todo_done_filename")
         let l:todo_done_filename = g:todo_done_filename
-    elseif expand('%:t') == 'Todo.txt'
+    elseif expand('%:t') =~# 'Todo.txt$'
         let l:todo_done_filename = 'Done.txt'
     else
         let l:todo_done_filename = 'done.txt'