(piqi) Add initial support for universal extensions
(piqi) Add initial support for universal extensions

Before this change, Piqi extensions could be applied only to type definitions:
records, variants, enums, lists and aliases.

Now, in addition to type definitions, they can be also be applied to fields,
options, functions, function parameters and imports.

alavrik/piqi-erlang,sanmiguel/piqi-erlang