Custom rendering for headers/blocks

[zhaoshenzhai]

From RyotaUshio in #27:

By the way, it might be helpful to let users choose how the heading/block links are rendered.
For example, maybe some people would like Definition (foo) rather than ^Definition (foo) or title > ^Definition (foo).
But I'm not sure how this should be realized. It's just a rough thought.

Enable this feature in settings.ts and update getMathLink accordingly.