added option to disable ffmpeg bindings because they can be hard to install
added option to disable ffmpeg bindings because they can be hard to install

WangDequan/pyvision,weiliu89/pyvision,WangDequan/pyvision,cvondrick/pyvision,weiliu89/pyvision,cvondrick/pyvision,WangDequan/pyvision,weiliu89/pyvision,WangDequan/pyvision,cvondrick/pyvision,cvondrick/pyvision,weiliu89/pyvision