Please add "HENCE" to parse-datetime

[jesse wilson]

Since you've already got "AGO", it makes a great deal of sense to add
"HENCE" to parse-datetime so we can change the date like this:

date -d "2 years hence"

Hence is definitely the antonym of ago, so this is a good idea. I
tried to write the patch myself, but I found parse-datetime.c to be
too complex for me to be able to figure out quickly.

Thanks,

Jesse