Customizable root indicators #1

Given a subdirectory of e.g. a git repo, the get_root function will tell you the root of that repo. Pretty straightforward goal, and there are some tests already.

For this issue,

1. Modify get_root to take an iterable (typing.Iterable[str]) of indicators. Using this type, list, set, and tuple should all work.
2. Default to the current ROOT_INDICATORS
3. Add a test that shows the customization works
4. Submit a PR!

Quick development instructions

1. git clone clone https://github.com/python-packaging/tuber
2. cd tuber
3. make venv
4. source .venv/bin/activate
5. make test (runs unittest)

Make sure the code is formatted (make format) and types are good (make lint) before submitting a PR to save iteration time waiting on CI.