Fix prev pointer of emph->first_child
Split process_emphasis into two functions

Makes the code more readable.
Remove redundant code

next/prev pointers are handled by cmark_node_free.
Optimize emph insertion

Avoid unnecessary malloc/free if opener is removed.
Add local variable for closer inline
Clarify code in insert_emph
Merge pull request #227 from nwellnhof/process_emph

Optimize and clarify process_emph