search_dates: problem with numeric incomplete dates

Issue may be in part related to #846

I have trouble to make search_dates recognise numeric incomplete dates correctly, such as 05/2020 or 2020.

1. In the first case, the 05 is recognised as day (and not as month, even though DATE_ORDER is DMY). Even more interesting is that the "month" in the datetime object depends on previously detected months.
   --> incomplete data matching should recognises the DATE_ORDER argument (thus recognising this as month) or date_formats could be supplied to search_dates, so that %m/%Y can be defined as date format.

2. the year is recognised, the day is set according to PREFERED_DAY_OF_MONTH, but the month is set the months recognised earlier in the text.
   --> would it be possible to have a PREFERED_MONTH_OF_YEAR option?

Text1: "test 01.05.2020 test. test 06/07/2020. test 05/2020. test 2020"
Result1: [('01.05.2020', datetime.datetime(2020, 5, 1, 0, 0)), ('06/07/2020', datetime.datetime(2020, 7, 6, 0, 0)), ('05/2020', datetime.datetime(2020, 7, 5, 0, 0)), ('2020', datetime.datetime(2020, 7, 1, 0, 0))]

Text2: "test 01.05.2020 test. test 06/10/2020. test 05/2020. test 2020"
Result2: [('01.05.2020', datetime.datetime(2020, 5, 1, 0, 0)), ('06/10/2020', datetime.datetime(2020, 10, 6, 0, 0)), ('05/2020', datetime.datetime(2020, 10, 5, 0, 0)), ('2020', datetime.datetime(2020, 10, 1, 0, 0))]

The first two dates are correct, the the third and fourth are not.

Thanks a lot :)

Start at the search_dates entry point and reproduce the Text1 and Text2 examples from the issue. Trace how incomplete numeric dates use DATE_ORDER, PREFERED_DAY_OF_MONTH, and previously detected months, then define behavior for 05/2020 and 2020 and verify that the reported results no longer depend on earlier dates.