Decreasing option for Vector's sort member function

[nathan-russell]

Currently sort does not have a parameter controlling the sort direction (ascending vs. descending). I have an update which would provide this option, changing the signature to sort(bool decreasing = false), as is the default in base::sort.

Although it is trivial to do this from client code for NumericVector and IntegerVector with std::sort(..., std::greater<T>()), that is not the case for CharacterVector (#419), so I think an internal implementation is warranted. If there are no objections, I can submit a PR for further review.

[coatless]

This would be great as the documentation for #419 in the FAQ can be switched from noting a flaw to providing an actionable solution. (Though, it might be also helpful to highlight that the use must be via the internal helper instead of stl.)

[nathan-russell]

@coatless Agreed -- I think this will provide a suitable workaround for a fair number of situations where #419 emerges as a problem, but the STL / string_proxy incompatibility is definitely something that should be documented.