Add subsec methods to Duration

[botahamec]

The standard library's Duration type has two useful methods:

• subsec_micros
• subsec_millis

This has the benefit of returning a u32 instead of a Option<i64>. I suggest the same three methods be added to chrono 0.4. I see we already have subsec_nanos in the 0.4.x branch, although it wasn't released yet. So this proposal just adds the two remaining methods.

Also, for consistency, should this method be called subsec_nanoseconds instead of subsec_nanos? That's what's used in DateTime.

[djc]

Let's be consistent with the recent addition and std over the older APIs. Happy to review a PR for this!