time.format is not respected for numeric values

[KarlAmort]

I have data with unix timestamps. I tried to use the time.format option to convert these, but as it turns out, the format option is used only as a last resort in Scale.Time::parseTime

Two problems:

1. It appears to me that it is unexpected behaviour to silently ignore an explicitly set configuration and use the built-in heuristic.
2. The check for numeric values appears to be too broad as there's an unlimited amount of numeric date & time formats that Moment cannot parse.

[etimberg]

Fixed in #2087