Support for multi-pass encoding

[escaped]

Many encoders provide multi-pass encoding for higher image quality and compression ration (eg. VP9). This is currently not supported and would be a nice addition for some use cases.

[ajeema]

if you want to use multipass, although there are much better methods that are faster, a tweak would be to reuse the first pass log file.

[escaped]

yeah, that's basically why it isn't supported. Currently we only do one pass without being able to reuse the logs for a second pass :) Pull Requests are welcome!