Add option to customize the hashed password field

[fibrasek]

First of all, awesome project!

When working on a project I found the necessity to give other name to the hashed password field, so I thought of writing this PR.

[BlakeWilliams]

Thanks for opening this!

Overall it looks good, but I had one question about how to organize the new option in terms of functions.

[BlakeWilliams]

What are your thoughts on splitting this into two functions? The first could have the same name/arity as the original method and the second could match on your field name?

e.g.

def hash_password(changeset) do
  hash_password(changeset, field_name: :hashed_password)
end

def hash_password(changeset, %{"field_name" => field_name})
  # existing logic in PR
end

That would also let us focus the documentation for each logical case.

[fibrasek]

That looks even better and would not introduce breaking changes related to the arity, nice!

I'll make these changes :)

[fibrasek]

@BlakeWilliams sorry about taking some much time to make those changes, but they're done :)