Extend inja template context with specific blocks of parsed comments

[bkryza]

Currently inja templates can only use the raw comment string of an entity such as class or method. Since libclang supports comment introspection and parsing:

https://clang.llvm.org/doxygen/group__CINDEX__COMMENT.html

it should be possible to provide access to individual comment blocks through inja for instance:

{{ element.comment.brief }}