RPN83P: RPN calculator for TI-83+ TI-84+ inspired by HP-42S

[bxparks]

The latest version (v0.12.0) of RPN83P is available at https://github.com/bxparks/rpn83p/releases. Download the rpn83p.8xk file and transfer it to your 83+ or 84+ calculator. This version should be compatible with v0.11.0, no data should be lost.

The primary purpose of this release is to improve some rough edges of the TVM functionality:

* support C/YR (compoundings per year)
* add "dots" to P/YR and C/YR when they differ from the default value of 12
* synchronize the END/BEG flag of RPN83P with the END/BEGIN flag of the "Finance" app of the 83+/84+ (finally)
* change default initial guesses of IYR1 and IYR2 to -50% and 100% respectively, supporting negative interest rates by default
* improve accuracy of I%YR very close to 0%
* display new status message "TVM Calculated (Multiple)" when multiple solutions are known to exist, but only one solution was found
* bug fix: fix incorrect logic for determining if commas are allowed in the middle of the input buffer

Unless major problems are found, I plan to cut v1.0 on top of this version in the near future.