Add support for durations.

[myusuf3]

To change something like this:

354h22m3.24s // :S

Into something like this:

2 weeks 18 hours 22 minutes 3 seconds

or various breakdowns like minutes or just hours.

[dmitshur]

This came up in the past as:

• Add support for converting duration to HH:MM:SS #52
• count time delta #48
• Support humanizing durations #24

[dustin]

Perhaps it would be good to understand why go only rolls up to the nearest hour. I suppose if we're approximating, we can throw away precision and be fairly loose with what a day is. This seems consistent with the idea of humanization.

Formatting is the hard part here. I suspect we'd want a data-driven language for formatting strings, e.g., one would like to express the example above as:

humanize.Duration("%y %m %w %d %H %M %S", d)

where the variables, e.g. %d expands into one of three string segments based on cardinality:

0: ""
1: "1 day"
n: n + " days"

The formatting table is fairly straightforward, but space consumption might be slightly tricky. In our case here, we'd want to consume any whitespace around the zero case, as it is effectively a lexical black hole absorbing the space made for time units that aren't relevant for its case.

An optional operator might cause the 0 case to be treated as the n case (or just supply a modified expansion table). This depends on how interesting such a case is.