Add the Sereal binary serialization format

Add detection for the Sereal binary serialization format. In addition to
printing out the format itself we'll also print the version of the
document, and the type. Currently there's 3 types, uncompressed, and
non-incremental/incremental Snappy compressed. See the link to the
Sereal specification.

Using the testcase I added to the Sereal repository in
https://github.com/Sereal/Sereal/commit/35372ae01d with this patch we'll
print out:

    $ ~/local/bin/file /tmp/sereal-v*; ~/local/bin/file --mime-type /tmp/sereal-v*; for f in /tmp/sereal-*; do echo $f: && hexdump $f | head -n 1; done
    /tmp/sereal-v1-plain.srl:          Sereal data (version 1, uncompressed)
    /tmp/sereal-v1-snappy-incr.srl:    Sereal data (version 1, compressed with incremental Snappy)
    /tmp/sereal-v1-snappy-nonincr.srl: Sereal data (version 1, compressed with non-incremental Snappy)
    /tmp/sereal-v2-plain.srl:          Sereal data (version 2, uncompressed)
    /tmp/sereal-v2-snappy-incr.srl:    Sereal data (version 2, compressed with incremental Snappy)
    /tmp/sereal-v2-snappy-nonincr.srl: Sereal data (version 2, compressed with incremental Snappy)
    /tmp/sereal-v1-plain.srl:          application/sereal
    /tmp/sereal-v1-snappy-incr.srl:    application/sereal
    /tmp/sereal-v1-snappy-nonincr.srl: application/sereal
    /tmp/sereal-v2-plain.srl:          application/sereal
    /tmp/sereal-v2-snappy-incr.srl:    application/sereal
    /tmp/sereal-v2-snappy-nonincr.srl: application/sereal
    /tmp/sereal-v1-plain.srl:
    0000000 733d 6c72 0001 6678 6f6f 6f66 666f 6f6f
    /tmp/sereal-v1-snappy-incr.srl:
    0000000 733d 6c72 0021 1909 780c 6f66 526f 0003
    /tmp/sereal-v1-snappy-nonincr.srl:
    0000000 733d 6c72 0011 0c19 6678 6f6f 0352 0000
    /tmp/sereal-v2-plain.srl:
    0000000 733d 6c72 0002 6678 6f6f 6f66 666f 6f6f
    /tmp/sereal-v2-snappy-incr.srl:
    0000000 733d 6c72 0022 1909 780c 6f66 526f 0003
    /tmp/sereal-v2-snappy-nonincr.srl:
    0000000 733d 6c72 0022 1909 780c 6f66 526f 0003