Single range support

We now support single part ranges, as per:

https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests#Single_part_ranges

Multi-part ranges are not implemented because there have been far too
many bugs in this area.

It interacts with compression by selecting a byte range from the
compressed stream. Since the compressed stream is fixed, the results are
consistent.

README.md

@@ -10,7 +10,7 @@ support for:
   - brotli, if you have the external compression binary available at pack time
   - does not yet support Transfer-Encoding, only Accept-Encoding/Content-Encoding
 - etags
-- ranges (TODO)
+- ranges
 
 The workflow is as follows:
 - (optional) build YAML file describing files to serve
@@ -18,3 +18,13 @@ The workflow is as follows:
 - create `htpack.Handler` pointing at .htpack file
 
 The handler can easily be combined with middleware (`http.StripPrefix` etc.).
+
+## Range handling notes
+
+Too many bugs have been found with range handling and composite ranges, so the
+handler only accepts a single range within the limits of the file. Anything
+else will be ignored.
+
+The interaction between range handling and compression also seems a little
+ill-defined; as we have pre-compressed data, however, we can consistently
+serve the exact same byte data for compressed files.