Add new IANA-allocated options from draft-muks-dns-filtering

[muks]

These new options are from the "DNS EDNS0 Option Codes (OPT)" registry in the "Domain Name System (DNS) Parameters" registry group.

https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml

Result of running the EDNS unittests on the edns-filtering branch:

[muks@p5 ~/tmp/dnspython/tests (edns-filtering)]$ python ./test_edns.py 
......................
----------------------------------------------------------------------
Ran 22 tests in 0.002s
 
OK
[muks@p5 ~/tmp/dnspython/tests (edns-filtering)]$ 

I have manually checked the Sphinx-generated HTML documentation in a web browser.

BTW, I've also added a call to unittest.main() at the end of test_edns.py. It does not affect the running of utest.py from the Makefile, and allows the unittests in test_edns.py to be executed separately.

Snippet of the result of running utest.py in the branch:

testEDEExtraTextLanguageOption (test_edns.OptionTestCase.testEDEExtraTextLanguageOption) ... ok
testEDEOption (test_edns.OptionTestCase.testEDEOption) ... ok
testEDEOption_from_wire (test_edns.OptionTestCase.testEDEOption_from_wire) ... ok
testEDEOption_invalid (test_edns.OptionTestCase.testEDEOption_invalid) ... ok
testFilteringContactOption (test_edns.OptionTestCase.testFilteringContactOption) ... ok
testFilteringDBOption (test_edns.OptionTestCase.testFilteringDBOption) ... ok
testFilteringOrganizationOption (test_edns.OptionTestCase.testFilteringOrganizationOption) ... ok