Improve Date and DateTime parsing

[dstengele]

Insight does not use a standard Date format for Date and DateTime attributes but rather the formats configured for display in Jira itself. Previously we had hardcoded formats which was removed in #7, instead returning the date strings directly and leaving the parsing to the user.

It would be preferable to either

• add a new config option to the Insight object to set the Date and DateTime formats in use or
• try to get the used formats directly from Jira and use them.

[dstengele]

With version 8.4 Insight finally uses ISO8601 as Date format: https://documentation.mindville.com/display/ICV86/Preparing+for+Insight+Version+8.4#PreparingforInsightVersion8.4-General-Dateandtimerequestandresponsetypes