Feature request: <ROT with numbers addition>

**Is your feature request related to a problem? Please describe.**
I just encountered a puzzle which switched the shift cipher from 0-9A-Z to A-Z0-9 affecting the results of the shift operation. Used the custom shift cipher at decode.fr.

**Describe the solution you'd like**
I thought it would be feasible to have an option either for custom character set for rotation similar to how there is currently custom substitution. Maybe an additional checkbox to allow numbers to be behind/in front of the alphabet as mentioned above.

Thank you in advance!