add bm-lifo-previous and bm-lifo-next function.
that means goto previous or next bookmark in LIFO order .
(that is, most recently set ones come first, oldest ones come last)

bm-show bm-show-all bm-next and bm-previous support bm-in-lifo-order option
and old bm-next and bm-previous are renamed to bm-common-next bm-common-next

fixbug of bm-show when no "\n" at end of line

add feature in README.md
     * Cycle through bookmarks in all open buffers in LIFO order (my frok version support)
bm-bookmark-add if bookmark already exists on this line ,update
timestamp of this bookmark,
Merge pull request #5 from jixiuf/master

add bm-lifo-previous and bm-lifo-next function.