Add remover option

[andresz1]

This PR adds a new option that is useful when you want to replace special characters rather than removing them. As the regex option name is called remove I thought that remover could be a good name but I'm not sure (no breaking changes).

Example:

slugify('Çumra.fdf', {
    replacement: '-',
    remover: ' ',
    lower: true
});

// output: cumra-fdf

Any feedback would be appreciated, thanks in advance.