New features in right prompt

I have updated README.md and added some features to the RPROMPT:
* <time<date ```POWERLINE_RIGHT_A="date"```
* <time<exit-code-status+icon ```POWERLINE_RIGHT_A="exit-status"```
* <time<mixed ```POWERLINE_RIGHT_A="mixed"``` <-- exit code status, if non-zero, or date
* <time ```POWERLINE_RIGHT_A=""``` <-- default

Variable ```POWERLINE_RIGHT``` changed to ```POWERLINE_RIGHT_B```

What do You think?