Support multi-unit Duration expressions

If one were to specify date/time as an expression, the options today are quite limited. For instance if one wants to specify ``today+11h45m``, it doesn't work. One needs to specify the exact fraction ``today+11.75h``. Ideally taskwarrior would support date/time expressions that can add or subtract the scales of year, month, days, hours, and seconds. So something like this:

``today+1y2mo3d1h4m``