Add "poster" as an acceptable attribute of <video>
Add "poster" as an acceptable attribute of <video>

This commit adds support for the "poster" attribute, marking its value
as being a URI.

ordbogen/html5lib-python,dstufft/html5lib-python,mindw/html5lib-python,html5lib/html5lib-python,mindw/html5lib-python,alex/html5lib-python,alex/html5lib-python,mgilson/html5lib-python,ordbogen/html5lib-python,mgilson/html5lib-python,ordbogen/html5lib-python,dstufft/html5lib-python,mindw/html5lib-python,mgilson/html5lib-python,html5lib/html5lib-python,dstufft/html5lib-python,gsnedders/html5lib-python,html5lib/html5lib-python,gsnedders/html5lib-python,alex/html5lib-python