Allow user to configure plugin settings.

Came up with a quick fix for [this issue](https://github.com/ntpeters/vim-better-whitespace/issues/8).

Each setting can now be configured in a `.vimrc`. I've tinkered with each of them individually to test if it works, and it does.

Inspiration for the function came from [nathanaelkane/vim-indent-guides](https://github.com/nathanaelkane/vim-indent-guides/blob/master/plugin/indent_guides.vim#L31-L39).