Revisit delta time formats

[hollasch]

timeprint should be able to track the next greater unit automatically, left to right. Possibly try out a new experimental delta token that does this automatically. Try new delta symbol -. This would yield the following syntax:

                   %-['kd][0]<U>[.[#]]
                      -v-  v  v  --v-
Numeric Format --------'   |  |    |
Optional leading zeros ----'  |    |
Units ------------------------'    |
Decimal Precision -----------------'

How to handle the case where the next greater unit was equal or smaller than the current? Consider suppressing the interpretation of the current code (printing it literally), or insert an error indicator.

As an example, this:

timeprint --now --time 20181029T160000 %_Dd %_d0H:%_h0M:%_m0S

becomes this:

timeprint --now --time 20181029T160000 %-Dd %-0H:%-0M:%-0S

More intuitive and less error-prone. An sketch of an erroneous format string:

timeprint --now --time 20181029T160000 %-Dd %-0H:%-0D:%-0S
15d 23:<ERROR: %-0D>:2718

[hollasch]

Alternative: lowercase characters denote context-aware modulo, uppercase versions are always full values. In addition, if a greater or same unit is specified after a lesser or same unit, then just reset the prior modulo to none.