Add support for writing M8 files

[whitlockjc]

Without the ability to write M8 files, this library isn't very useful. While this isn't technically hard, there are places in the M8 files (specifically the Song file) where data is unknown (but not empty) and where data is unused. Updating the parsers to record this data so we can write the data back exactly as we found it will be required. So improvements to the parser and parser debugging (#1) will be required.