[dateparser] Add feature add_prefer_month_year

[adnan-awan]

Resolves https://github.com/scrapinghub/dateparser/issues/1042

To do:

• [x] Add tests for this feature
• [x] Update the docs

[adnan-awan]

Thanks @Gallaecio for useful suggestion!

I've added the suggested test. Would you please review it again?

[anuj1508]

@Gallaecio Just checking on when are you planning to push a new release which includes above changes?