Add `to-month` argument to `download` entrypoint

[FrancescoCasalegno]

Currently the download entrypoint requires to specify a "from date" in the format of YYYY-MM. Then, the command will download every paper from (inclusive) the 1st day of that month until today (inclusive).

We need to fix this behavior as follows.

• to-month mandatory argument should be added to the entrypoint.
• The command will download every paper from the 1st day of from-month until the last day of of the month before to-month.
  E.g. if from-month=2021-07 and to-month=2021-09, then the command should download papers published on any of the following days:2021-07-01, 2021-07-02, ... 2021-08-30, 2021-08-31.
• The command should check if to-month is the current month or before, and otherwise raise an error exit. E.g. calling to-month=2021-02 on 2021-02-01 or later is fine, but calling it on 2021-01-31 or before will fail.