Add hooks for `pyenv-init`

This can allow plugins to implement their shell functions into user's shell.
For example, install following scripts as `${PYENV_ROOT}/pyenv.d/init/detect.bash`
will detect the changes of the Python version in pyenv.

```bash
cat <<EOF
_pyenv_hook() {
  if [ -n "\$OLD_PYENV_VERSION" ] && [[ "\$OLD_PYENV_VERSION" != "\$(pyenv version-name)" ]]; then
    echo "pyenv: Python version has been changed: \$OLD_PYENV_VERSION -> \$(pyenv version-name)"
  fi
  OLD_PYENV_VERSION="\$(pyenv version-name)"
};
if ! [[ "\$PROMPT_COMMAND" =~ _pyenv_hook ]]; then
  PROMPT_COMMAND="_pyenv_hook;\$PROMPT_COMMAND";
fi
EOF
```