Add ability to have a per-model prefix

It would be nice to be able to specify a per-model prefix, eg:

class User(models.Model)
    id = AutoHashId(primary_key=True, prefix='us_')
    # ids like us_3n3b3hjbjh4

class Project(models.Mode)
    id =  AutoHashId(primary_key=True, prefix='pr_')
    # ids like pr_j5f7l3hg5k5l

[nshafer]

Not a bad idea, and you’re not the only one to think of it. I just found @pegler has created a fork with that functionality. I’ll review his work.

[nshafer]

OK, I have just removed the callable support for now so that it doesn’t hold up a new release. I did think it was a little superfluous since it’s called in pretty much the same context as just setting the string, but I figured there was a use-case I wasn’t aware of or thinking of. I’ll take another PR once it’s more defined.