Ever since version 2.0, when parsing a journal file, jrnl would not use
the custom date format string anymore. Instead, it relied on the
dateutil library to get the parsing right. This change was made to allow
people to change their date format without having to manually change
their file. However, this broke some existing date formats like
%d.%m.%Y, as it would falsely interpret the month as day and vice versa.
This commit adds some tests to catch this error and fixes it by trying
to parse the dates using the custom format first, only falling back to
dateutil when needed.
