Create --max-chars argument

[Benjamin-Lee]

I'm using xkcdpass for a website and hit the following message:

Value above must be no more than 20 characters in length.

It doesn't look like there's an argument to allow this. I'm thinking that a --max-chars argument might be useful here for systems with requirements like this.

[MalteKiefer]

I need this too.
Is there an easy way to implement it?