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