[YoutubeDL] Add --client-certificate option

[dirkf]

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Description of your pull request and other information

Some sites only allow access to HTTPS connections that offer a valid client certificate. In some cases, the client certificate may be built into the firmware of a client device. While such certificates are typically not accessible without jail-breaking the device, it would be desirable to allow this means of authentication as well as username+password, etc.

This PR adds an option --client-certificate whose value should be the filename of a single certificate file containing the certificate and private key (as described in the Python library documentation for the ssl module under 'Combined key and certificate').

[dirkf]

The new test case test_check_clientcertificate failed for Python >= 2.7.8 because yt-dl sets the HTTPS client to validate the server certificate, and this failed with the test HTTPS server from test/test_http.py because self-signed certificates don't pass.

If 'nocheckcertificate': True is passed along with the clientcertificate option in the test_check_clientcertificate test case, the test succeeds.

The tests for HTTPS in test/test_http.py don't check that HTTPS connections work in general, but only that the nocheckcertificate option works. A similar test-case without that option fails, but this will happen unless the server certificate has a valid path to a root CA. Unfortunately, arranging proper SSL certificates for testing seems to be a hopeless task.