Add project specific configuration via local or remote `pyproject.toml`

[lagru]

Closes #17. Closes #23.

The idea is to have three sources of configuration, in order of priority:

• Local TOML file specified with --config option
• Remote pyproject.toml at the last given revision
• Default configuration in _config.py

For the release notes:

Add support for configuration files either via a remote `pyproject.toml` or a local TOML
file given with the new option `--config`. This file allows the configuration of templates,
ways that changelist matches labels, finds summaries, as well as logins to ignore in the
contributor section.

[lagru]

Do you mind updating the README with configuration instructions?

Of course, I just didn't get to that part yet. ^^

[stefanv]

Looks good to me! As soon as there's a pointer from the README to the example config, we're good to go.