Friendlier api for functions like cqlengine.AbstractQuerySet.only to use variable arguments

I think it would be nice to change the implementation of functions like `defer` and `only` to accept a variable number of arguments. Unfortunately this would be backwards incompatible so I understand this may not be accepted.

Let's say the new version of `only` is defined like this:

Instead of

Use cases:
1. Passing in one field.
Old code

New code

2. Passing in multiple fields.
Old code

New code

3. Passing in a list of fields
Old code

New code

Perhaps with the exception of the last example, I think the new api is easier to use and less prone to mistakes.