Added replace_last and corresponding tests to list of standard filters.

Hi @dylanahsmith. I'd like to contribute the following change to liquid. Would you be able to do a code review?

The change is to add a standard filter called "replace_last" which simply replaces the last occurrence of a string with another string.

Although this of course could be implemented as a custom filter, I thought it was basic enough that it should be included in the list of standard filters.

I also added some corresponding tests.

Best regards,
Mark Benson