Publish Intel SPIR-V Extensions (#57)

* update cl_intel_device_side_avc_motion_estimation

* switch to HTML extension specs for Intel extensions

cl_intel_required_subgroup_size
cl_intel_subgroups
cl_intel_subgroups_short

* add Intel SPIR-V extensions

cl_intel_spirv_subgroups
cl_intel_spirv_media_block_io
cl_intel_spirv_device_side_avc_motion_estimation