support rufus-scheduler duration notation

[tobeportable]

Just added the ability to define a month with "M" and a year with "Y"

• for the M. cf. https://github.com/jmettraux/rufus-scheduler/blob/master/lib/rufus/sc/rtime.rb
• for the Y -> Y not

[henrypoydar]

Hrm. Maybe we should figure out why "mo" instead of "mos" doesn't work before merging.

[tobeportable]

I am sorry this comment was supposed to be a separate issue as it is unrelated to this merge.