Allow to restrict the list of issues to migrate

[SebastianZ]

To gain better control over which issues are ported, it should be possible to provide a search query as parameter for the script, e.g. -q, --query.

Examples:
Issues created within the last year.

-q "opened-after:today-365"

Issues modified within the past three months having a test case.

--query "modified-after:today-90 Test=case-available"

Issues containing the text "1000":

-q "\"1000\""

Sebastian