StrtoTime: Allow using a custom date format

This is a great addition! I do wonder if we should fallback to strtotime() when parsing the date fails. Maybe provide that as an option? "Fallback to strtotime() if the date could not be parsed.". What do you think?

If you have applied the Tamper "String to Unix Timestamp" to the source "date" and set the date format to "d/m/Y" and then import the following CSV:

title,date
Foo,2020-09-01
Bar,01/11/2011
Qux,01-10-2011

This is the result after import (using the latest dev releases of Feeds and Feeds Tamper):

I've added the fallback option. 🙂

The fallback option seems useful, the more flexibility the better.

I'd still personally recommend that users avoid using strtotime() blindly since as per the original issue, it can accidentally convert it into an invalid date, so your feed should ideally have a standardized format to convert from (however having the fallback option still wouldn't hurt for cases where the dates are entered manually).

All that being said, the code and tests looks good! RTBC.

Thanks for the review! I merged the code.