index.md support

[robhrt7]

Currently SourceJS supports Markdown only in special sections, like

<markdown>
# Hello
World
</markdown>

But to follow other tools and creating more pleasant experience for Markdown fans, we need to add full support of index.md, instead of index.src.

The task is to compile Markdown file to SourceJS compliant Spec, which is not so hard. Possible realization:

Add checking for index.md on request -> compile MD to HTML with existing libs -> enhance HTML with SourceJS specific markup.

[robhrt7]

Already working on it https://github.com/sourcejs/Source/tree/operatino/markdown-support

[robhrt7]

Done #100