rc: add CRF ratecontrol

1. add a parameter of "--crf" to the command line

2. modify the running branches of rateControlStart, using "if(isAbr)" instead of
"switch (cfg->param.rc.rateControlMode)", for the logic of classifying the
combination of multiple ratecontrol methods is very complex, it is not only
based on rateControlMode, so porting x264's way looks feasible.

3. add crf method into x265