Implement custom recurrence periods
Created by: wsdfhjxc
Okay, so here's the implementation of custom recurrence periods, as discussed in #25 (closed). Also, there are some i18n changes, and the simplified plurals make sense in English, but in other languages, with complex numbering rules, it might look weird. If there is ever need for more context-aware translations in regard to numbers, then I think it still can be done with the help of i18next.
@ransome1 If you have any suggestions for improvements, or requests for some other changes, let me know.