Reworked markdown.clj; See in-file comments for details.

Implemented most of Pegdown's LinkRenderer class in Clojure                                                                                                                                                                                
https://github.com/sirthias/pegdown/blob/master/src/main/java/org/pegdown/LinkRenderer.java                                                                                                                                                
Notably the default-handlers format is the same as the custom handlers format used for tests,                                                                                                                                              
and most of the proxy/proxy-super code is gone. Trying to ensure perfect parity with the                                                                                                                                                   
Java class to ensure output is the same for benchmark and comparison purposes.                                                                                                                                                            
If/when the Pegdown devs create an interface for LinkRenderer,                                                                                                                                                                             
it would enable better/faster Clojure-Java interop, e.g. reify could be used instead of proxy                                                                                                                                              
See https://github.com/sirthias/pegdown/issues/132