Because chrono was depending on an older version of time, and we don't
actually need it with recent versions of chrono.
Relates-To: #1188
Alernative-To: #1190
This patch uses Chrono for explicit date or datetime parsing, only using
humantime for its relative time parsing. The following formats are accepted:
1. Full RFC3339 parsing, requiring an explicit timezone
2. `YY-MM-DD`, defaulting to time `00:00:00` for the given date in the
local time zone
3. `YY-MM-DD HH:MM:SS` in the local time zone
Fixes#631, #794