Todo: add multi-range support

[glebec]

Currently send-seekable only supports single range requests (e.g. bytes=2845-8473 or similar). A server accepting range requests is supposed to handle multi-range requests too (bytes=128-932,1043-2834 or similar). This puts send-seekable in the awkward position of throwing an error on such requests.

The most fundamental limitation preventing this, for now, is the reliance on the range-stream package. If that limitation can be overcome, remember to use range-parser's new (as of v1.2) feature of combining multiple overlapping ranges into single ranges. Also note that multiple ranges can be requested out of order.