- Sheets can have any number of rows
- Sheets have a max number of cols (26)
- Text and Float are the only types
- Expressions are forth style: A2 B3 + C2 A2 - +
- Ranges work via ':', for example: A2:C6 +

- Available expression functions: +, - . \*, /, POW, MIN, MAX, SQRT, ROUND, SUBSTR, UPPER, LOWER, CAPITAL, SUM


TODO:
  - Write expression parser
  - Add methods for running expressions
  - Figure out relative reference logic (when you copy/paste an expression)
  - File save/save-as, custom format?
  - File load: custom format, csv
  - Ability to add/remove/rename sheets
  - Improve TUI
  - Make manpage
  - Make Makefile