add a few helpful scripts for generating asciidoctor attributes

[bashbaug]

gen_extension_attribs.py accepts a text file of enums, APIs, and types added by an extension and generates asciidoctor attributes for them. This is useful when authoring EXT or vendor-specific extensions.

gen_feature_dict.py accepts a text file of OpenCL C features and generates asciidoctor attributes for them.

I'll leave this as a draft for a bit while we figure out what to do with it.