Allow negative offsets in `hms` parsers

[vspinu]

Currently

> hm("-04:00")
[1] "4H 0M 0S"

[jfinke]

Agreed. I currently have a list of vectors that have positive and negative offsets.

`

hm("-12:53")
[1] "12H 53M 0S"
hm("12:53")
[1] "12H 53M 0S"
`