add a few feature of $(svn_prompt_info)

* var ZSH_THEME_SVN_NUVC_IN_DIRTY

  when set to 0, the NOT UNDER VERSION CONTROL data is not regarded as dirty data.

* var SVN_DIRTY_COUNT

  show the count of dirty data. example: 

  `$ZSH_THEME_SVN_PROMPT_DIRTY="%{$fg[red]%} ${SVN_DIRTY_COUNT%}+%{$reset_color%}"`