Add support for customisable TODO keywords

[alex0]

About the issue of custom TODO keywords, if it was for me I’d trash the current implementation for something based on tags and namespaces:

• the user creates tags like:
  • #task/todo, #task/doing, #task/done
  • #bug/open, #bug/closed
  • etc
• namespaced tags like these have a little button next to them that change them by cycling between tags of the same namespace

If order is crucial, then introduce a special property that these tags would have to specify the order:

task/todo.md
next:: #task/doing

Possible UI implementation:

Advantages:

1. You can open #parent/child page to have an overview in Linked References for example of #task/todo.
2. You can open #parent page for the same overview of all #task, #bug etc.
3. You can further specify by adding other levels like #bug/closed/wontfix that still count as #bug/closed
4. The three points above but in queries, for example (and [[project]] [[task/todo]])