enhance custom replacement; Add support for "black-listed" words #20
- keep old behavior for single character replacement
- add custom replacement for string to string|char|'' // '' ~= remove

ex.:

slug = getSlug('*Foo and Bar or Baz', {
    custom: {
        'and': 'UND', // replace word
        'or': '', // remove word
        '*': 'star' // replace character
    }
});
console.log(slug); // Output: starfoo-und-bar-baz