Pulling external file changes to a tangled org-babel src block
is surprisingly not a well-implemented feature.  This addresses that.

Any block that has :tangle <fname> will compare the block with
that external <fname>.  When a diff is detected, 1 of 5 actions
can occur:

  1. External - <fname> contents will override the block contents
  2. Internal - block will keep the block contents
  3. Prompt - The user will be prompted to pull external changes
  4. Diff - A diff of the <fname> and block contents will be produced
  5. Custom - A user-defined function will be called instead.

These 5 options can be set as the default action by changing the
`org-tanglesync-default-diff-action` custom parameter.  Otherwise
individual block actions can be set in the org src block header
e.g. `:diff external` for pulling external changes without
prompt into a specific block.

This package also provides a hook when the org src block is
being edited (e.g. via `C-c '`) which asks the user if they
want to pull external changes if a difference is detected.
The user can bypass this and always pull by setting the
`org-tanglesync-skip-user-check` custom parameter.

Since v1.1, a "watch" mode has been added that does not require you
to be within the org buffer to sync changes to the tangled blocks,
but can now instead modify directly from the external tangled buffer
and the changes will be synced back to the org file in the background.
The `org-tanglesync-watch-files` needs only to be set a list of org
files with tangled blocks, and `org-tanglesync-watch-mode` needs to
enabled globally.