Use the most advanced `gpt-4o` to replce deprecating `gpt-4-vision-preview`

Hi @zyocum, Thanks for your great work on this amazing PDF to Markdown tool. I just tested it, and it's really awesome! Because the GPT-4-Vision model is being deprecated, I've confirmed that `gpt-4o` can be directly used to replace it, it'll be great if this patch can be merged as the default model, it's more powerful with more competitive pricing and performance. Pricing comparison: - gpt-4o $5.00 / 1M input tokens, $15 / 1M output tokens - gpt-4-vision-preview $10.00 / 1M input tokens, $30 / 1M output tokens Deprecation info reference: > On June 6th, 2024, we notified developers using gpt-4-32k and gpt-4-vision-preview of their upcoming deprecations in one year and six months respectively. As of June 17, 2024, only existing users of these models will be able to continue using them. - https://platform.openai.com/docs/deprecations/2024-06-06-gpt-4-32k-and-vision-preview-models