`search_dates` doesn't support custom date formats

[noviluni]

It would be nice to add support fot his, but I don't know how feasible it is.

Original comment: #833 (comment)

[noviluni]

Maybe we could move the date_formats to the settings instead of using a parameter. I didn't think this to much, but it could make sense to change this API (first adding support for both formats and later deprecating the second one).

Let me know your opinions.

[stuartthomson]

This is something that would be very useful to me too. Would be willing to contribute a PR if the design could be agreed.