Support creating shims of virtual environments in `envs` (#103)

etc/pyenv.d/rehash/envs.bash

@@ -0,0 +1,11 @@
+virtualenv_list_executable_names() {
+  local file
+  shopt -s nullglob
+  for file in "$PYENV_ROOT"/versions/*/envs/*/bin/*; do
+    echo "${file##*/}"
+  done
+  shopt -u nullglob
+}
+if declare -f make_shims 1>/dev/null 2>&1; then
+  make_shims $(virtualenv_list_executable_names | sort -u)
+fi