Additional string filters

I found myself needing to remove the last character of a string without knowing the length of the string in advance. At first, I thought I could use slice, but ran into the behavior mentioned in https://github.com/Shopify/liquid/issues/522. I then tried a few other things, including reversing the string, removing the first character, then reversing again. However, I didn't get the behavior I was looking for.

I think a few new filters would have made my issue easier to solve, as well as be good complementary filters to the ones which already exist.

clip (or some other name): Similar to slice, but allow for negative indices for people who want a Python-style method.
str_reverse: Reverses a string
replace_last, remove_last: Perform the same as their counterparts, replace_first and remove_first, except they obviously start at the end of the string and read backwards.

Merely having the str_reverse filter would make all the others above achievable (though a bit cumbersome) with existing filters, but I think all four of the filters would be great additions to the toolbox.

Start by reviewing the existing slice, replace_first, and remove_first filters, then read issue #522 for the slice behavior that motivated this request. Done requires a settled API and semantics for the proposed filters, with the requested string operations covered.