Fix increasing anzu information in mode-line issue(#13)

anzu.el assumes anzu information is first element of mode-line-format.
anzu.el decides there is no anzu information if first element of
mode-line-format is not anzu information. But this is sometimes wrong.

If some element is consed mode-line-format during anzu information showing,
anzu.el mistakes that there is no anzu information and cons anzu information
again to mode-line-format. This causes increasing anzu information.

So I fixed anzu.el checks whether there is anzu information in mode-line-format
by 'member' function and remove anzu information from mode-line-format by
'delete' function.

Cc: #13