Support excluding buffers by name, to support Buffergator and fix #48

This pull request changes how numbers.vim checks buffers against g:numbers_exclude.

Previously, buffers were only checked by their filetype against g:numbers_exclude. This meant plugins like Buffergator, which don't set the filetype of their buffers, could not be excluded, as detailed in #48.

numbers.vim now checks buffers by both filetype and name against g:numbers_exclude. It also adds "[[buffergator-buffers]]" (the name of Buffergator buffers) to the default value of g:numbers_exclude.

These two changes now means numbers.vim plays nicely with Buffergator, and similar plugins.