Merge: lib/markdown: improve performance a bit

Using benchmarks/markdown:

* nitmd_master is before
* nitmd_md is this now
* nitmd_md_fast is with #1973 in addition
* the 3 same avain with with --semi-global
* 2 Java libs: txtmark, markdown4j

![bench_markdown](https://cloud.githubusercontent.com/assets/135828/13517453/b008a7fe-e191-11e5-80c8-d17cd85df6be.png)

There is still a lot of performance improvements to do anyway.

Pull-Request: #1972
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>