Add support for negative integers with leading zeros and floating point numbers

[JonathanLindsey]

This adds support for

1. Negative integers with leading zeros
2. Floating point numbers

Notes for floating point numbers

1. This assumes the display has exactly 4 digits.
2. Only supports use of the colon (not decimal points) for now. Implementation theoretically allows for decimal points to be added later without breaking changes. - I can't get the displays I bought to show decimal points.
3. When -1 < num < 1 and leading_zero is false, superfluous leading zeros may be shown. This is documented with a note that this behavior may change in the future.
4. Implementation does not include length and pos arguments but these can theoretically be added in the future without breaking changes.

[JonathanLindsey]

There are some things I want to change/fix before this gets merged.

• [x] Instead of passing the DecimalType enum as an argument, I'll change the function name to showNumberFloatColon(). This is less typing for the user.
• [x] Fix an integer overflow bug causing floating point numbers like 700 to get messed up in the current implementation.
• [ ] "Support" numbers that are too high or low to fit on a 4 segment display by showing HIGH and LO__ messages.
• [ ] Support Inf, -Inf, and NaN by displaying _InF, -InF, and nAn_

If you think LO__ or nAn_ should be right justified, let me know.