Support for VP8 and VP9 video codec

[ming]

Add support for VP8 and VP9 video codec using ​libvpx.

During our testing, for the same bitrate, VP8 quality seems to be better than H264, but the encoding speed is noticeably slower. For VP9 however, as documented by several other sources as well [1][2], the encoding speed is much much slower, even though we have followed the recommended setting for "Real-time CBR Encoding and Streaming" from ​their doc here. Because of this, VP9 is currently by default disabled. To enable it, you can set PJMEDIA_HAS_VPX_CODEC_VP9 compile-time macro.

[1] ​https://blogs.gnome.org/rbultje/2014/02/22/the-worlds-fastest-vp9-decoder-ffvp9/
[2] ​https://www.streamingmedia.com/Articles/ReadArticle.aspx?ArticleID=111334

Steps to use VP8/VP9:
Download ​libvpx. Build and install. Libvpx will then be autodetected by PJSIP configure script (alternatively, you can specify --with-vpx option).