Disable all features if var is set

[delphinus]

I want to disable dwm.vim at only specified windows. For example, denite.nvim opens a complecated & multifunctional window, so I want to make denite manage the window, not by dwm.vim.

This PR introduces a buffer-specific variable: b:dwm_disabled. When you want to exclude some windows under dwm.vim's, you can achieve it with using autocmd.

autocmd FileType foo :let b:dwm_disabled = 1
" or
autocmd BufEnter some-bufname :let b:dwm_disabled = 1
" and so on

Perhaps, #70 can be realized by this PR and similar methods above.