Add writing_role configuration for database connection switching

[joshuajansen]

This PR fixes #18 by adding a writing_role setting that gets used to wrap the save_records call in the rollup method in. If not configured it'll call and return block directly by default.

Usage could look something like this for the implementing codebase:

Rollup.writing_role = :writing

ActiveRecord::Base.connected_to(role: :replica, prevent_writes: true) do
  Invoice.group(:project_id).rollup("Revenue per project") { |i| i.sum(:total_amount_cents) }
end

Which would then fetch the data from the replica and then switch to writing the rollup to rollups using the writing role.

I'm very open to a less monkey-patchy way of testing this! And of course to any feedback on the implementation in general.