Allow empty replacement

[abdonrd]

My use case is create usernames without separator.
Following the documentation: https://github.com/sindresorhus/slugify#separator

I have tried with:

slugify('BAR and baz', {separator: ''});

And I expect this:

'barandbaz'

But I get an error:

SyntaxError: Invalid regular expression: /{2,}/: Nothing to repeat

[sindresorhus]

Sure. PR welcome.