set listchars and showbreak to prettier characters

an edited version of https://github.com/fphilipe/dotfiles/blob/65c9434927a81567e5119e1ffc7965fc2ea9d72e/vimrc#L95-L97
customize word characters using autocommands

including a TODO for customizing characters in the default file type (where 'filetype' is empty)

autocommands based on https://github.com/fphilipe/dotfiles/blob/65c9434927a81567e5119e1ffc7965fc2ea9d72e/vimrc#L517-L519