Attach the underlying raw records to Sector structures; add a .raw exporter for

getting the MFM/FM/GCR stream.

doc/using.md

@@ -282,14 +282,22 @@ FluxEngine also supports a number of file system image formats. When using the
   
   - `<filename.ldbs>`
 
-	Write to a [LDBS generic image
-	file](https://www.seasip.info/Unix/LibDsk/ldbs.html). **Write only.**
-  
-  - `<filename.d64>`
+  Write to a [LDBS generic image
+  file](https://www.seasip.info/Unix/LibDsk/ldbs.html). **Write only.**
 
-	Write to a [D64 image
-	file](http://unusedino.de/ec64/technical/formats/d64.html), commonly used
-	by Commodore 64 emulators. **Write only.**
+  - `<filename.d64>`
+  
+  Write to a [D64 image
+  file](http://unusedino.de/ec64/technical/formats/d64.html), commonly used by
+  Commodore 64 emulators. **Write only.**
+
+  - `<filename.raw>`
+
+  Write undecoded data to a raw binary file. **Write only.** This gives you the
+  underlying MFM, FM or GCR stream, without actually doing the decode into
+  user-visible bytes. However, the decode is still done in order to check for
+  correctness. Individual records are separated by three `\\0` bytes and tracks
+  are seperated by four `\\0` bytes; tracks are emitted in CHS order.
 
 ### High density disks