Add support for relative date parsing in args

[remyleone]

Description

When we use date we want to be able to parse it like "-3d" or "+3d" relative to the current date.

How this functionality would be exposed

I should be able to fill a date argument using either an absolut dateime or a relative one

scw ... from=2020-07-08T12:00:00Z  <= ISO 8601 /  RFC 3339
scw ... from=+3d <= 3 day in the future
scw ... from=3d <= 3 day in the future
scw ... from=-3d <= 3 day in the past

We should at least handle d, h, m, s

As relative time will probably cause problems with golden we should consider adding (if needed) a core.ExtractNow(ctx) that can guarantee constant result in tests

References

Version

Version 2.0.0-beta.4+dev
BuildDate unknown
GoVersion go1.14.4
GitBranch unknown
GitCommit unknown
GoArch amd64
GoOS darwin

[remyleone]

@jerome-quere The problem with adding a core.ExtractNow is that it will create an import loop because the core package already import args.