add unite.vim support

added unite.vim support to show list of memos. below variables are
introduced.
    - g:memolist_unite
        specify using unite.vim or not (defualt 0)
    - g:memolist_unite_source
        specify a source of unite.vim (default 'file')
    - g:memolist_unite_option
        specify options when a list of memo is opened with unite.vim

add syntax highlight to README