Add more controls to Gradio interface

[zebradots]

The Gradio interface to rembg is great, but it lacks controls for several frequently used variables. For example, the alpha matting erosion size often must be adjusted for best results, but it can't be changed from the graphical user interface.

For anyone who uses the Gradio interface, it would be very helpful to be able to adjust these variables on the fly. It would be great to have sliders or checkboxes for the following variables:

• Alpha matting (checkbox)
• Foreground threshold (slider)
• Background threshold (slider)
• Erosion size (slider)
• Only mask (checkbox)
• Post process mask (checkbox)

While it may be possible to pass these values as text or JSON arguments, doing so isn't user friendly.