Support almost-ISO8601 patterns in date filter ISO8601 config

- Use an array for date filter internal field @parsers instead of a single value.
- Under the ISO8601 format, use first the joda ISO parser (accepting only a 'T'), then add another parser for **almost ISO** formats (accepting a space)

Feel free to add other patterns that you also consider ISO8601 valid :see_no_evil:

Fix LOGSTASH-180 and LOGSTASH-1582