500,000 byte file size limit

Markdown Writer FX presently enforces a limit of 500,000 bytes on the open file:

```
file is too large for editing.
File size is 583,245 bytes.
Maximum is 500,000 bytes.
```

However it operates buttery smooth on 400,000-byte Markdown files. I suspect it will do fine with much larger files.

Please consider removing the file size limit restriction.