Support multiple datasets in a repository

[timwis]

They should be inside folders named after the dataset, each with their own config file, e.g.

.gitsheets/
  students.toml
  teachers.toml
students/
  1
  2
teachers/
  1
  2

[themightychris]

To add to that, each sheet config should have a configuable root and path template, e.g.

[gitsheet]
root = "slate/students"
path = "{{ graduation_year }}/{{ status }}/{{ student_number }}"

This will provide a lot of flexibility for integration gitsheet storage into any project, and opens the door to a submodule-compatible solution for referencing arbitrary remote gitsheet datasets