Extending %m+% to weeks and days

[larmarange]

So far, %m+% works only with periods containing only years and months. Therefore it's not possible to do something like today() %m+% period(c(1,6),c("month","day)".

Would it be possible to extend %m+% in a such way that previous example would be equal to today %m+% period(1,"month") + period(6, "day")?

I would also allow to write more generic code when the unit is not known.

[vspinu]

We need this one way or another to fix #235. So I am willing to give it a stab. The only issue is that we will need a convention, do we sum years/months first or HMS first. I think years/months first is a bit more meaningful because the function is called %m%:)