Add a pay period command

[Christopher Roussel]

It would be nice to have a period command that shows all activity for a pay period. There should be a setting which indicates the start date of the (current) payroll period and a setting indicating the payroll period length.

Some use cases:

Some companies operate on a calendar month basis. Difficulty: easy.

Some companies operate on a monthly payroll, but start at some point in the middle of the month (e.g. 2nd Sunday). Difficulty: medium.

Many companies operate on a fortnightly basis (biweekly for the US folks). Difficulty: easy.

A number of companies operate on a weekly basis. Difficulty: easy. Already done.

Other companies may have stranger payroll periods, so we should allow them to create a custom schedule (e.g. every 3 weeks).

Ideally, the payroll period should be created during the setup process with a sensible default.