Enhancement: Allow Grouping by File

[matkatmusic]

It would be great if the list of TODOs included subgroups based around filenames.
Example:
TODO:
--File1.cpp
----TODO: Fix this thing here:
----TODO: Fix this other thing here that needs fixing
--File2.cpp
----TODO: This also needs fixing
----TODO: we should probably fix this, but maybe after the initial release

instead of just a massive list that doesn't sort them by filename.