Support plugin exclusion by buffer name

Previously, items in the g:numbers_exclude list were only checked
against the filetype of a buffer. This worked great with, say,
NERDTree, which sets the filetype of its file tree buffers to
"nerdtree". However, plugins like Buffergator don't set the filetype of
their buffers. They do, however, set the buffer title.

numbers.vim now checks the items in g:numbers_exclude against both the
filetype of a buffer, and its name.

This commit also adds "[[buffergator-buffers]]" to the default
g:numbers_exclude list. These two changes should cause numbers.vim to
exclude Bugffergator windows by default.