ISO8601 support

[matthuszagh]

I attempted, and failed, to use chrono to parse a number of ISO8601 datetime formats. Looking through existing issues (e.g., #244), it appears that chrono is more in accordance with RFC3339 than with ISO8601. While these standards have some formats in common, they are not the same (FYI this website provides a nice illustration of the commonalities and differences).

chrono clearly states at the very top of the README that

• Chrono strictly adheres to ISO 8601.

which seems a bit misleading. I think it would be useful to specify both what chrono does support and what its goals are. The first point is self-explanatory, but I think it's useful to elaborate on the second point. For example, does chrono aim to fully support ISO8601, or only RFC3339, or a subset, or both?

As an aside, it would be really nice if chrono did aim to fully support ISO8601. For example, I'm currently attempting to use chrono in an application that stores publication dates (as well as more granular datetimes). Publication dates often only specify a year. I could permit chrono to be used by specifying more specific datetimes, but this is inaccurate because it implies a false precision. I.e., I might only know that a book was published in 1988, so to specify the datetime to the nearest second is incorrect.

FYI, this point was already made in the linked issue as a comment. But, I believe it's worth making this its own, separate issue.