New Package Name Format Verb

This release adds the %k formatting verb to output the package name for the function or method of stack.Call. The unadorned verb outputs only the last segment of the package import path, while the %+k variety produces the full import path of the package.

Note that %k was chosen in part because one of this package's forks (https://github.com/getlantern/stack) had chosen it for the same purpose. %p was considered, but the fmt package already uses that for formatting pointer values, and we wanted to avoid potential confusion with that use case.