I think we need to be careful of reading too much into past texts from our modern perspectives. Feminism as a movement was completely unheard of in the time of Sophocles. Yes, he does create and base a play around a strong female figure, but, as #2 points out, Antigone has many faults of her own and is not a perfect character in many ways. However, at the same time, by giving Antigone a privileged position and showing her rebellion against authority, she could be viewed as a feminist predecessor.
Hmmm. Well, Sophocles clearly created a strong woman in the form of Antigone. She knows her mind and she's willing to speak it. She has a strong moral compass, and it's not just words for her. She is willing to stand up to authority (in this case her uncle and king) in the name of justice. On the other side, she is stubborn and strong-willed. She is dismissive of anything or anyone who doesn't think as she does. She's convinced she's right and is unwilling to consider any other options. Looking at all of this, I'm not sure this is particularly exclusive either to a feminist or even women in general. Sophocles could not have been making a statement directly toward modern feminism; however, he does seem to say that women are capable of moral conviction and thinking and reasoning and action, qualities which a male-dominated society hasn't always wanted to attribute to its women.