[DOXIA-584] Rewrite .md and .markdown links to .html

[jmones]

This implementation supports rewriting the following types of links:
(link to a)[a.md]
(link to xxx section in a)[a.md#xxx]
(link to a)[a.markdown]
(link to xxx section in a)[a.markdown#xxx]

[jmones]

I'm improving the solution a little bit more. Will create another pull request.

[hboutemy]

notice: you can do this by just updating your branch, without closing this PR :)

if you can add a little unit test, that would be awesome

[jmones]

Hi Hervé. Now you can look at it. I implemented it as a regex and added some unit tests, as requested.
Don't hesitate to request any modifications.

[hboutemy]

great work!!!
please merge the PR I just sent on your clone for unit tests formatting
then squash the commits
then I'll merge

[jmones]

Done. I hope it's ok (no much experience on this).
Please note that I had to update flexmark dependency in POM, as I needed newer version (i hope that's ok).
Finally, do you have plans to release soon?

[hboutemy]

merged, thank you: great job
new Flexmark version still requires Java 7, then it's perfect
on releasing, there is no precise plan for the moment: I'll review in a few weeks