New: Date to Time plugin

Issue cleanup.

#7, #8 confirmed #6 patch is working so moving to RTBC

Two remarks:

1. I wonder if it would made sense to merge this plugin with the existing strtotime plugin? We could make setting a date format optional in this case.
2. There needs to be a functional test as well to ensure that configuration gets saved.

This plugin worked really well as there is ambiguity with importing dates in the format dd/mm/yyyy

Thanks @tostinni for the work on this. If I had a better understanding of writing tests I'd sort this out!