Add filler class

As of yet untested.
Correct and generalise filler.

It can now be used directly as the engine for `fill-slice`.
Split `today` script.

The fetching of the schedule from the gateway is now done in `today-raw`.
This leaves the formatting and prettifying to `today`.
Make rubocop more happy.
Rename programs to make rubocop happy.
Update readme to use new names.