parse_datetime should support "@" syntax

[philolo1]

We would like parse_time to support "@" syntax, that indicates the seconds passed from the "Thu Jan 1 00:00:00 UTC 1970".

See related issue: uutils/coreutils#5177

I am intending on working on this issue.

[Benjscho]

I believe parse_datetime::parse_datetime::from_str supports parsing "@" syntax in this section. Just added a new test to verify: #31

[philolo1]

@Benjscho you are correct, i also added some tests (did not see your pr until now). #32