allow for dmenu options

[tjaartvdwalt]

Hi, thanks for a great project.

I normally increase font size when using dmenu, something like: dmenu -fn 'monospace-16'
looking through the code it seemed the logical thing to do was to configure $menu in $XDG_CONFIG_HOME/dmenfm/preferences using the command above.

This works great the first time I launch dmenfm, but when I launch it a second time, the menu configuration line has disappeared from the preferences file, and my dmenu command reverts to the default.