should detect externally-modified files

When running more than one instance of TKDO, or editing task files in both TKDO and vim at the same time, it is far too easy to clobber changes from one program in the other. TKDO should detect when files have been modified and reload them instead of saving over them.

The situation could be handled by any of these:

  - Poll the task files and reload automatically whenever one changes.
  - Check task files when the cursor is on a relevant item, and ask to reload if appropriate.
  - Detect changes while saving, reload, then attempt to proceed anyway.
  - Abort the current operation and tell the user the file was modified, then reload.

I'm kind of leaning toward the last one for now, because it's the safest and easiest. Most operations are just a few keypresses, so it usually doesn't cost much to repeat the action after reloading.