Add fish prompt changing (#475)

bin/pyenv-sh-activate

@@ -245,7 +245,20 @@ if [ -z "${PYENV_VIRTUALENV_DISABLE_PROMPT}" ]; then
   case "${shell}" in
   fish )
     if [ -z "${QUIET}" ]; then
-      echo "pyenv-virtualenv: prompt changing not working for fish." 1>&2
+      cat <<EOS
+functions -e _pyenv_old_prompt              # remove old prompt function if exists. 
+                                            # since everything is in memory, it's safe to
+                                            # remove it.
+functions -c fish_prompt _pyenv_old_prompt  # backup old prompt function
+
+# from python-venv
+function fish_prompt
+    set -l prompt (_pyenv_old_prompt)       # call old prompt function first since it might 
+                                            # read exit status
+    echo -n "(${venv}) "                    # add virtualenv to prompt
+    string join -- \n \$prompt              # handle multiline prompts
+end
+EOS
     fi
     ;;
   * )