I see two use cases for extensions:
- those developed locally that I want to use with gtimelog installed using `sudo apt-get install gtimelog` or `pip install gtimelog`; it makes sense to put extensions into, say, `~/.conf/gtimelog/extensions` or maybe `~/.local/share/gtimelog/extensions`.
- those intended to be redistributed; it makes sense to upload these into PyPI as packages like `gtimelog-foobar` and install them using `pip install gtimelog-foobar`.

In neither case it makes sense to look for extensions in the gtimelog package directory.

Setuptools entry points are perfect for use case #2, this code (adjusted to use a different directory) should be fine for case #1.

Maybe we should support both?

Maybe extensions should be configurable via gtimelogrc?