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)