Make path separator configurable for dir prompt

Now the path separator can be configured by setting
`POWERLEVEL9K_DIR_PATH_SEPARATOR`

powerlevel9k.zsh-theme

@@ -456,6 +456,7 @@ prompt_custom() {
 }
 
 # Dir: current working directory
+set_default POWERLEVEL9K_DIR_PATH_SEPARATOR "/"
 prompt_dir() {
   local current_path='%~'
   if [[ -n "$POWERLEVEL9K_SHORTEN_DIR_LENGTH" ]]; then
@@ -503,6 +504,10 @@ prompt_dir() {
     esac
   fi
 
+  if [[ "${POWERLEVEL9K_DIR_PATH_SEPARATOR}" != "/" ]]; then
+    current_path=$(echo "${current_path}" | sed "s/\//${POWERLEVEL9K_DIR_PATH_SEPARATOR}/g")
+  fi
+
   typeset -AH dir_states
   dir_states=(
     "DEFAULT"         "FOLDER_ICON"