Allow settings to be defined before including the variables file

[justinlocsei]

This makes the configuration variables in the variables.styl file act as default values that defer to any previously defined variables of the same name, which follows the convention used in the Sass version of Font Awesome (https://github.com/FortAwesome/Font-Awesome/blob/master/scss/_variables.scss).

[raulghm]

Thanks @justinlocsei