Exclude subtasks, add option to manually refresh from command palette

[harmtemolder]

I’ve added a setting that lets you restrict the plugin to only include tasks without an indent. It might not solve #17 completely. For example:

- [ ] root task
  - [ ] sub task
- not a task
  - [ ] another root task? or not?

The last task will not be included when “Only show root tasks” is on. I’m not sure if it should. What do you think?

Also, I’ve restructured the plugin a bit, for clarity. And I’ve added a command palette command to refresh all tasks, which also includes a notification.

[UCh]

This will be a good work-around for people like me that do heavy use of sub-tasks that are not self-explanatory without the surrounding context