Force the Python implementation of protocol buffers to be used.
Allow font size to be inherited in tables.

NewsBlur appears to use quirks mode when rendering item bodies in its iOS app,
and font sizes are not inherited in tables in quirks mode (see
https://developer.mozilla.org/en-US/docs/Fixing_Table_Inheritance_in_Quirks_Mode).
We forcibly enable inheritance so that NewsBlur's font size controls can affect
digest bodies.