Reorganize the grammar

- Use START_AMOUNT instead of "{" so it is parsed before TEXT
- Allow only one plan and make the structure hierarchical
- Add a header with version number
- Put the amount at the start of the line to simplify parsing and
  make it easier to align amounts

example

@@ -1,19 +1,20 @@
-# Full work plan {5000}
+takentaal v0.1.0
+#  {5000} Full work plan
 
 This is the description of the entire work plan.
 
-## First task {1000}
+## {1000} First task
 
 This is the description of the first task.
 
 This description has # ' " [] symbols and ü © Ð Latin-1 chars.
 
-- First subtask {500}
-/ Second subtask {500}
+-   {500} First subtask
+/   {500} Second subtask
 
 ## Second task {1000}
 
 This is the description of the second task.
 
-* First subtask {500}
-- Second subtask {500}
+*   {500} First subtask
+- {500} Second subtask