RGBFMT tool to fix code formatting

Modern programming languages tend to have auto-formatters. It would be worthwhile to have one for RGBASM as well, supporting users' various preferred styles.

Configurable options (not meant as a complete list):

- **Indentation:** tabs or N spaces
- **Max line length:** backslash line continuations could apply for long lines (0 = no limit)
- **Case:** lower or UPPER, separately configurable for:
- hex digits
- instructions/registers/conditions
- directives/keywords
- plus a way to special-case some (e.g. lowercase everything except `MACRO/ENDM` and `SECTION/ENDSECTION`)
- **Instructions:** some instructions have alternatives
- the optional `a` argument for `add`, `sub`, `cpl`, etc
- `ld [hli/hld]` vs `ld [hl+/hl-]` vs `ldi/ldd [hl]`
- `ldh [c]` vs `ld [$ff00+c]`
- `nz/nc` vs `!z/!c`
- **Blank lines:** configure how many blank lines to put:
- before global labels
- before new sections
- before other directives
- **Anonymous labels:** put `:` on its own line, or on the same line as the subsequent instruction
- **Macros:** indent macro invocations or not (or e.g. indent all of them except `mycol1macro`)