Add: prioritize update the priority

When there is already a priority in the line, we update it instead of adding
a new one

ftplugin/todo.vim

@@ -111,8 +111,12 @@ function! TodoTxtPrioritizeDecrease()
     normal! 0f)h
 endfunction
 
-function! TodoTxtPrioritizeAdd (priority)
-    " Need to figure out how to only do this if the first visible letter in a line is not (
+function! TodoTxtPrioritizeAdd (priority)
+    let line=getline('.')
+    if line !~ '^([A-F])'
+        :call TodoTxtPrioritizeAddAction(a:priority)
+    else
+        exec ':s/^([A-F])/('.a:priority.')/'
     endif
 endfunction