Stricter `datetime` validation

The [docs about the `datetime`](https://github.com/github/relative-time-element#datetime-string) attribute say:

> This must be a valid [ISO8601 DateTime](https://en.wikipedia.org/wiki/ISO_8601)

This statement is incorrect. Any value that can be passed to the [`Date` constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date#date_time_string_format) will work which presents a cross-browser issue because JS engines are inconsistent in which formats their `Date` constructor accept.

For example, the golang default string representation only parses in v8:

```bash
$ eshost -e "new Date('2009-11-10 23:00:00+00:00 UTC')"
#### JavaScriptCore
Invalid Date

#### spidermonkey
Invalid Date

#### v8
Wed Nov 11 2009 00:00:00 GMT+0100 (Central European Standard Time)
```

Also try [this fiddle](https://jsfiddle.net/silverwind/m425n0yg/) in multiple browsers.

How about adding an option attribute to pass in a validation regex into the element to validate the passed dates, when present? This would at least not make this issue missable by developers who only test in Chrome.