Close an existing vimfiler window properly when using multiple filetypes.

* Close an existing vimfiler window if -toggle option is provided.
* vimfiler#util#get_vimfiler_winnr returns an appropriate winnr
  even if a buffer has multiple filetypes.
* Compares filetypes by using a regular expression in
  vimfiler#init#_start.
Merge pull request #255 from shinnya/fix/get_vimfiler_winnr

vimfiler can't close an existing vimfiler when the vimfiler buffer has multiple filetypes.