Allow more control of stacktrace formatting.

Currently stacktraces are printed with `clj-stacktrace`, if available, or `clojure.stacktrace` otherwise. To use a different print function, the common approach seems to be to call `alter-var-root` on one of these in the lein profile -- a bit of a hack.

This lends some flexibility to stacktrace printing by adding `io.aviso.repl/pretty-pst` as another automatically searched option (like `clj-stacktrace.repl/pst+`), and defining a variable with which the user can override the built-in defaults.