Dithertron-generated NES ROMs not valid

[hxlnt]

To reproduce:

1. Navigate to Dithertron and select "NES (4 color, 240 tiles)."
2. Add a custom image or use the default image and click "Open in 8bitworkshop." Code and emulation output are shown as expected.
3. From the top left menu, select "Download > Download ROM Image."

The generated ROM does not open in Mesen (although other NES ROMs downloaded from, for example, C code samples work fine). Mesen shows "Error: Could not load file [filename.nes]." Since it won't load at all, I can't get any debugging info from Mesen.

I took a quick glance at the generated ROM in a hex editor, and I suspect there's a problem with code chunks being .orged where they're supposed to. Kinda looks like something's off.

Unfortunately, I don't have time to take a deeper look right now, so I thought I'd log an issue and see if @sehugg or anyone else has thoughts. I might be able to come back to this in a few weeks, too. :)

Thanks!

[hxlnt]

OK, just kidding, I made a PR to fix: #42

The issue was a lack of padding in the graphics chunk. Feel free to close this if the PR looks satisfactory.