~/.xmonad: add shortcut for spawning emacs in "writing" mode

e.g. start in `writeroom-mode`.

.xmonad/xmonad.hs

        ((mod4Mask, xK_b), spawnHere "firefox"),
        ((mod4Mask .|. shiftMask, xK_b), spawnHere "firefox -private-window"),
        ((mod4Mask, xK_e), spawnHere "emacs"),
        ((mod4Mask .|. shiftMask, xK_e), spawnHere "emacs -f writeroom-mode"),
        ((0, xF86XK_AudioMute), spawn "amixer set Master toggle"),
        ((0, xF86XK_AudioLowerVolume), spawn "amixer set Master 5%-"),